It's pretty clearly purely for the sake of optics.
In particular, note that this measure did not come up when we had a Democratic majority in the Senate and a Democratic President so it stood at least some chance of being passed.
There's a pretty simple reason for that. The US Constitution (article 1, Section 8) says:
The Congress shall have Power
17: To exercise exclusive Legislation in all Cases whatsoever, over such District (not exceeding ten Miles square) as may, by Cession of particular States, and the Acceptance of Congress, become the Seat of the Government of the United States [...]
Although it hadn't been ceded or named yet when the Constitution was written, that's describing Washington DC--the seat of the Government of the United States.
As a state, Washington DC would have its own government that was not entirely subordinate to the federal Congress1. So, Congress would no longer be able to "exercise exclusive legislation in all cases whatsoever" over Washington DC.
Washington DC is the one piece of land in the US that is prohibited by the Constitution from becoming a state. If they honestly wanted it to become a state, what they would have drafted would have been an amendment to the constitution.
So, they passed a bill that they not only know won't pass the Senate, but that (they know even better than we do) would be blatantly unconstitutional if it were signed into law.
So what we have is a bill that's pure, unadulterated "posing". Those who wrote and voted for it know full well that it won't become law, and more importantly realize that even if the Senate and President were amenable, it still couldn't become law, because it would be blatantly unconstitutional.
At the same time, they can use it for talking points during an election they clearly believe is going to be difficult. Rather than admit that the Senate ignored it because it was obviously unconstitutional, they'll try to push the notion that Republicans opposed it because they're anti-democratic, racist, etc. And, there's little question that to at least some extent, this will succeed.
On one hand, it's absolutely true that modern Republicans are extremely prone to believing certain conspiracy theories, even ones that are utterly silly and lack any factual support (e.g., Birthers).
It is, however, also true that modern Democrats are equally willing to believe equally silly nonsense with equal lack of factual support. In the case of Democrats, this takes a somewhat different form. It's largely a belief that if I espouse a measure I claim will benefit marginalized group X, then any opposition to it can stem only from hating X2.
Purely in theory, you can sort of get around the constitutional problems by writing the law not to turn all of Washington DC into a state, but to instead just "shrink" Washington DC to include only the capital building, white house, etc., so you still have some minuscule area of land that's officially the seat of government, but the actual residents of the area all live in the new state.
Dealing with that at all well also requires a constitutional amendment though. The problem is pretty simple: under the 23rd amendment, Washington DC gets 3 electoral votes, just like it would if it were a state. If we shrink Washington DC so it has no residents, those electoral votes now represent...virtually nobody. Essentially the only people we could claim were residents and being represented by those electoral votes would be the first family, since they reside in the White House, which would almost certainly have to be considered part of the seat of government (and even that would be a change--the first family normally retain residency in their home state, so (for example) in the last election, Trump voted in New York.
So even if we skirt the primary constitutional issue itself, we immediately run into another that's equally problematic (i.e., also requires a constitutional amendment to fix).
- ...in contrast to its municipal government, which is subordinate to the Congress. From a legal viewpoint, the DC municipal government is basically a "hired hand" acting on behalf of congress. Congress essentially has veto power over any action taken by the municipal government. Congress can change the rules under which it operates or (in theory) could even abolish the municipal government and administer it directly. In contrast, a state government is an independent entity acting on its own behalf, with full power to make its own laws, even if those are contrary to the wishes of the federal government (e.g., legalizing drugs that are prohibited by the federal government).
- As opposed to, for just a few examples, thinking your measure will be ineffective, do more harm than good, benefit X but only at the expense of group Y who need help just as badly, provide minimal benefits at exorbitant costs, or simply falls within the range of powers granted to the states rather than the federal gov't.