Lifted from Wikipedia:
Strategic significance
IDF soldiers and Israeli settlers, 2009
In 1983 an Israeli government plan entitled "Master Plan and Development Plan for Settlement in Samaria and Judea" * envisaged placing a "maximally large Jewish population" in priority areas to accomplish incorporation of the West Bank in the Israeli "national system".[292] According to Ariel Sharon, strategic settlement locations would work to preclude the formation of a Palestinian state.[293]
Palestinians argue that the policy of settlements constitutes an effort to preempt or sabotage a peace treaty that includes Palestinian sovereignty, and claim that the presence of settlements harm the ability to have a viable and contiguous state.[294][295] This was also the view of the Israeli Vice Prime Minister Haim Ramon in 2008, saying "the pressure to enlarge Ofra and other settlements does not stem from a housing shortage, but rather is an attempt to undermine any chance of reaching an agreement with the Palestinians ..."[296]
The Israel Foreign Ministry asserts that some settlements are legitimate, as they took shape when there was no operative diplomatic arrangement, and thus they did not violate any agreement.[297][298][299] Based on this, they assert that:
Prior to the signing of the Egypt–Israel peace treaty, the eruption of the First Intifada, down to the signing of the Israel–Jordan peace treaty in 1994, Israeli governments on the left and right argued that the settlements were of strategic and tactical importance. The location of the settlements was primarily chosen based on the threat of an attack by the bordering hostile countries of Jordan, Syria, and Egypt and possible routes of advance into Israeli population areas. These settlements were seen as contributing to the security of Israel at a time when peace treaties had not been signed.[300][301][302]
Now, of course Israel has peace treaties with these neighbors, except for Syria and enjoys a large regional military advantage since they stopped receiving masses of Soviet arms, so the defensive justification is not as applicable as it was in the past.
(US State Dept map showing the neighboring countries wrt West Bank)
Leaving the others reasons and adding an influential pro-settlement group in the Knesset.
Do settlements aid or weaken Israeli military security, today?
That's a complex question. It would be remiss not to note that Hamas flourished in Gaza partially thanks to the absence of Israeli oversight. But on the other hand, it seems to be have the case that, on Oct 7th, 2023, most of the IDF's attention was on the West Bank, to assist settlers - War on Rocks 4m28.
Or, Economist:
The prime minister took this policy to a new level in building his current government: a coalition with extreme religious, ultra-nationalistic parties, which stated quite openly that Israel would never enable the establishment of a Palestinian state, give equal rights to the Palestinians under a one-state solution or stop the plundering of their lands through settlement-building. This policy led to most of the Israeli army being deployed to protect Jewish settlers in the West Bank, at the expense of protecting the border around the Gaza Strip.
Add to these reasons the ethno-religious motivations - Biblical Israel - given as justification by another answer (answer which also corroborates that some people's intent is to keep out a Palestinian state - "no good can come out of enabling the Palestinians to create a state of their own") and that probably covers most of it. Oh, and speaking of that answer's claim of "empty lands", take a look at the other SE.History answer re. 1967 events.
7 Things To Know About Israeli Settlements : Parallels : NPR
- What does Israel say about settlements?
The settlers and their supporters cite the Jewish Bible, thousands of years of Jewish history, and Israel's need for "strategic depth" as reasons for living in the West Bank.
They also note that Israel took the territory from Jordan, which has since relinquished its claim to the West Bank. Therefore, the settlers argue, there is no legal sovereign in the territory.
Some reading about one of the settlements, Ofra, by B'Tselem, a Jewish pro-peace organization.
Ofra was established within the context of an ongoing struggle between the
Gush Emunim movement, which was founded in February 1974, and the Israeli
Labor government, which opposed establishment of settlements on the Samarian
mountain ridge that was densely populated by Palestinians.13 Previous attempts
to establish settlements in the northern West Bank had failed, as the Israeli army
had removed the settlers. In light of this, Hanan Porat and Yehuda Etzion, two
of the movement’s leaders, decided to try a different tactic: rather than directly
confronting the government, carrying out settlement out cunningly and with no
media coverage.
In 1974, the army began to construct a base on Ba’al Hatzor Mountain, northeast
of Ramallah, on village lands of Silwad. In December 1974, Porat and Etzion
convinced the building contractor, Zalman Barashi, who was in charge of
fencing the base, to hire a group of Gush Emunim members. According to Hagai
Huberman, “there was now a possibility to sneak into Samaria ‘through the back
door,’ by belonging to a work group that was building the base on Ba’al Hatzor
Mountain.”14
On 20 April 1975, after five months of traveling daily to Ba’al Hatzor Mountain
and back, the workers from Gush Emunim did not return to Jerusalem when the
day’s work was over, but remained in the abandoned houses of the Jordanian
‘Ein Yabrud army base, some three kilometers southwest of the mountain.15 The
same evening, Porat met with the defense minister at the time, Shimon Peres,
and asked him not to evict the settlers. Peres agreed, provided that the site be
considered a “work camp” and not a “community,” and with the understanding
that the state would not allocate any budget for it. In other words, “The IDF will
neither help them – nor hinder them.”16
The government’s silent consent was exploited by Gush Emunim, which brought
more settlers to the site and named it Ofra. In December 1975, the Defense
Ministry recognized Ofra as “a workers’ camp for purposes of regional defense,”
and approved connection of the buildings on the site to the electricity grid.17
Shortly afterwards, Peres visited the site and stated that, “the time has come to
stop this charade about it being a work camp” and that Ofra should be given the
official status of a community. These comments appear to have been made with
the approval of then-Prime Minister Yitzhak Rabin.18 However, official recognition
of Ofra as a community was given only following the political revolution in Israel,
with the ascendency of the Likud government in 1977.19
* Googling it up does find hits for the "Master Plan", but I would be cautious about assuming too much without seeing some actual evidence of said plan. Things having to do with Israel often suffer from anti-Semitic conspiracy theories, including supposed Zionist blueprints that really never existed. Just because Wikipedia says it...
