There seem to be two aspects to this.
(1) On a local level, regarding the status of the primarily Russian-inhabited self-declared breakaway provinces/republics in Eastern Ukraine.
The Russian Foreign Ministry stated that it is their priority to preserve the status quo as of the Minsk agreements which structured the ceasefire following hostilities in Eastern Ukraine in 2014-2015.
In this context, the buildup of forces thus functions as a signal that if Ukraine attempts to re-fight the 2014-2015 actions, with or without western support, then the implication is that Russia has the ability immediately come to the defense of the self-declared breakaway republics in Eastern Ukraine.
The presence of these forces within Russia is widely interpreted as also presenting the possibility of an all-out invasion of Ukraine, e.g. to capture territory not already under Russian control. The same was said in 2014-2015, and not much has changed since then. The question asks what would be the benefit, and IMO, for a wider action, on balance there would not be any benefit, because Ukraine would not accept it without more serious fighting, while they did at the time grudgingly accept the relatively more limited outcome of 2015.
(2) On a larger level, in the view of the Russian side, all of this is happening in the context of NATO signaling expansion into Ukraine. Motivational context form the Russian side, on this topic, can be found in many statements to the press. Some of the most recent are linked and exerpted below -- from the Russian Deputy Foreign Ministers Ryabkov and Grushko on 18 and 20 December 2021:
First interview [Russian Ministry of Foreign Affairs]
Sergey Ryabkov: [...] The security situation in Europe, the Euro-Atlantic region and Eurasia has indeed greatly deteriorated recently. This has happened
because of a series of concerted actions by the United States and its
NATO allies, which, generally speaking, can be described as an attempt
to undermine Russia’s security and to create a hostile environment
around us. We cannot accept this.
Ukraine is in the focus of this policy. Ukraine’s decisions are not
independent but are subject to change in the situation. When the West
provides unconditional and unqualified support to Ukraine, certain
quarters in Kiev play up to the worst Western objectives and formulas.
And the possibility of Ukraine eventually joining NATO, which some
Ukrainian officials keep talking about, is categorically unacceptable
to us. We will do our best to prevent this.
Second interview [Russian Ministry of Foreign Affairs]
This situation did not just happen out of the blue.
Even after 2014, when NATO almost completely curtailed cooperation
with the Russian Federation and simply discarded the positive agenda
that had been achieved with such difficulty, we proposed specific
steps that could, if not improve the situation amid the destruction of
security mechanisms, at least achieve some de-escalation. That was our
response to NATO’s calls to take steps to lower tensions. We agreed.
And where has that got us? The line of contact between Russia and NATO
is extending. During the Soviet era, the contact was only along the
Turkish and Norwegian borders. Who created this line of contact and
now says they are concerned about Russia's activity? What, do we have
to tighten our belt now? Pull our forces back to the Urals?
Our approach is well grounded and based on the new reality. Even if we
compare it with 1997, when NATO and Russia signed the Founding Act,
which included NATO’s pledge not to deploy additional substantial
combat forces in the new member states on a permanent basis or change
its nuclear strategy, configuration of nuclear weapons and
infrastructure – these commitments are also in question now. NATO
Secretary General Jens Stoltenberg recently said the Alliance could
deploy nuclear weapons in Eastern Europe. This is a serious challenge
to the very foundations of European security. NATO used to play with
phrases such as “temporary deployment.” Now they are talking about a
fully sustainable and rotational presence. This, in fact, means a
permanent presence. All of this directly affects our security. If you
read the reports by the leading Western political science centres,
they frankly admit that NATO has created new vulnerabilities for
itself by moving its borders to the suburbs of St Petersburg. At the
same time, the distance from Tallinn to St Petersburg can be covered
by bike; NATO combat aircraft can reach St Petersburg in less than ten
minutes. This factor cannot be neglected. It must be taken into
account in military planning, and we will certainly do so.
NATO expansion has turned the Baltic region, which used to be one of
the most peaceful regions, into a theatre of military rivalry that
no one needs, least of all Russia. During the Cold War, NATO believed
it had one vulnerable spot – the Fulda Gap, a series of passes through
the hills the Warsaw Pact tanks could hypothetically use to reach the
English Channel. Now the alliance is concerned about the Suwalki
Corridor, a 65-kilometre-wide strip linking Poland with Lithuania,
squeezed between the Kaliningrad Region, a Russian exclave, and
Belarus. It connects NATO with its Baltic members, which fear they
could be cut off in the event of a conflict. Apparently, NATO’s
eastward expansion has compromised the alliance’s own security. If
NATO had remained within the borders our Western partners promised to
Mikhail Gorbachev, who would they have to defend themselves from now?
The whole expansion process was, in fact, a way to prove the
alliance’s relevance. But today, it is affecting fundamental security
interests. And when we talk with our Western partners, and they
complain about military activities on the border between Russia and
Ukraine, we reply: “Look at the map.”
In essenece, just as NATO views the encroachment of Russian forces with suspicion (and even alarm), the view stated by the Russian MFA is the mirror image.
One minor thing the drama did produce is statements from US and NATO that they would not, at this time, deploy their forces to aid Ukraine, if the 2014-2015 fighting were repeated. Establishing this may have been one of the goals.