I don't know the exact reason why, but what i've been told is that once you're known to have overstayed you're effectively banned from getting another visa to come back in for an amount of time, with that time starting after you are documented as leaving.

I'm pretty sure there's something that when you overstay a visa, get deported or have some kind of issue you basically can't get another one for 10 years. If you accrue a debt like medical debt and then leave without paying, the same is true afaik - can't get in until you pay, and I think the 10 year wait starts when it's paid in full.

We had a lawyer for my wife's process and she guided us. It's been a while, but we submitted our documentation within the timeline of her visa, and the lawyer told us she's fine to stay even though the visa expires before you really get a response beyond "we received your paperwork" confirming delivery. I think you get a case number with USCIS very early as well. It's been over 5 years for us at this point and my memory of the specific steps is not great. My wife handled the process for the most part since she couldn't legally work at all until she got a work authorization a couple of years in.

I think we also were given a warning to not leave after getting married until she got a green card because then they wouldn't let her back in or give her work authorization until the sponsorship cleared. There were also many extra delays and extra steps added to the whole process during the first trump administration, like the very long wait for work authorizations was not typical (and an obvious attempt at stopping poor immigrants.) I'm not going to lie, I was worried outside of the biden administration about them throwing a wrench into it. Many people mid-process this past year had their visas revoked and were deported even during their final interview or after the interview while going to the swearing in ceremony despite doing nothing wrong, being in the correct process, and already being cleared. Their problem was that they were somali or haitian or from some other vulnerable country with the "wrong" skin color.