Trying to force though "the deal"
This is the main reason for this vote.
The prime minster is hemmed in and is trying to show that there is no other option but to accept "her deal", as already negotiated with the EU. She knows that there is not a majority for "no deal" in the parliament. So the vote is a way to show to everyone that the majority of parliament do not want a no deal, vis a ve parliament (or more to the point the pro-brexit MPs) should accept her deal. It's trying to prove to the MPs that say "we should leave without a deal" that they are in the minority.
Context
This needs to be viewed in the context of what is currently happening. There are lots of factors at play here, some big ones include (not an exhaustive list or we'd be here all day):
- The current government is a minority government without an overall
majority in the house, it is only in power currently because of a
loose arrangement with the DUP. This is so loose that the DUP has
voted against the government several times! So the government, even
if it could force ("whip") all of it's MPs to vote with it, still
cannot make parliament accept the current deal on the table on its
own.
- The government has lost 2 major votes votes on its main policy
("Brexit"). Under normal circumstances this would cause the
collapse of the government and a general election.
- Earlier last year, the Prime Minister barely scrapped a no
confidence vote.
This has again highlighted that the government cannot produce a
majority. Again, Under normal circumstances this would cause the
collapse of the government and a general election.
- The government tried increase its majority with its snap election of
2017. The idea being that if it had a larger majority it could better sideline the more extreme wings of the party (such as the ERG).
This tactic failed miserably resulting in the Conservative party
losing its majority. So the Conservative party really doesn't want
an election because it's afraid it will lose it or it'll make
matters even worse (again!).
- The DUP could withdraw its support to the government; this would
cause the collapse of the government and a general election. The
DUP doesn't want an election though because it now has a lot of
power. The DUP is a "Northen Ireland Unionist" party. It is usually a small player in wider UK politics but has a strong vested interest (obviously) in the border between northern and southern Ireland.
So at the moment the UK government is basically after something it can call a win in any shape or form, even if it's not binding at least it shows progress.
You can see form above just how tentative the current governments grip on power is. The current state of affairs is totally unprecedented. At any other time this government would have collapsed a long time ago or at the very least the prime minister would have resigned. Why has none of this happened? Because there simply isn't enough time to hold elections, etc. before 29th March.
The government never wanted to have this vote. It's been forced into by the circumstances. But without the government it's hard to make votes legally binding in the current circumstances. So the government is trapped by parliament and parliament is trapped by the government. Eventually (dear god hopefully soon!) one will have to give.
A vote cannot rule out Brexit, legally
The other thing here is that there is primary legislation(an act of parliament) that says the UK will leave the EU on the 29th March.
The only way to revoke this is with more primary legislation. This vote is not primary legislation. Revoking article 50 would require a new Act of Parliament. The EU has said the UK can revoke article 50 whenever it wants, the UK act of parliament says this isn't possible. This was only a vote on the original Brexit bill.
Unless a deal is agreed by the 29th March, then the legal default is no deal, no matter how many votes in parliament say that it's not true, the law says no.