Otdfbt

As reported in the bbc article available here: https://www.bbc.co.uk/news/uk-politics-47742395

The UK government has split the 'deal' into the actual agreement and the non-legally binding political declaration - an explanation of which is available here:
https://www.bbc.co.uk/news/uk-politics-46303751

Since it seems more like a statement of intent than anything else, why would the government need parliament to vote on it at all? Couldn't they just get the actual agreement through, then say "You've had your meaningful vote. Deal with it."

Because the UK domestic law says so, in section 13 of the European Union (Withdrawal) Act 2018:

13  Parliamentary approval of the outcome of negotiations with the EU

(1)  The withdrawal agreement may be ratified only if—
 (a)  a Minister of the Crown has laid before each House of Parliament—
 (i)  a statement that political agreement has been reached,
 (ii)  a copy of the negotiated withdrawal agreement, and
 (iii)  a copy of the framework for the future relationship,
(b)  the negotiated withdrawal agreement and the framework for the future 
relationship have been approved by a resolution of the House of Commons on a motion 
moved by a Minister of the Crown,

Article 50, which governs how countries leave the EU, states that in the two years after triggering it the EU will negotiate how that country will leave with consideration given to the future trading relationship.

So the UK tried to include a political declaration about that future relationship.

That failed spectacularly, twice. So now the government is just trying to pass the part that sets up the transition period and trade negotiations, with would technically deliver brexit as the UK would leave the European Union. May has already said she is going and will leave the mess for someone else to clear up, so her focus right now is securing her legacy by being able claim she delivered.

As for the meaningful vote, to avoid a constitutional crisis and further legal action they will have to eventually vote on the future relationship, but that will be somebody else's problem.

The Speaker of the House of Commons has said that he won't allow a motion that is substantially the same as a previously defeated motion to be moved by the Government. This is their workaround, splitting it into two motions to be voted on separately.

It may be the case that the government does not need the vote on the political declaration, but they hold the vote anyway as another "advisory" vote. It would certainly look bad that they hold "continuous meaningful votes until one passes" by doing constant fiddles like this.