Democrats pushed for this by using Covid as justification. I think it was illegal in some cases because they did it without legislative approval.
It worked so well for Ds that they made it a permanent thing. If Rs opposed they were accused of denying people the right to vote. That's a typical D strategy.
Some Rs have adopted a strategy of "If you can't beat them, join them" so they are now trying it themselves. The problem is they were late to the game and they have a lot less to gain.
The fact is that we had a very good checkpoint for voting.
That checkpoint was that voters showed up to a voting location and voted on election day.
By removing that checkpoint you are left with situations like have been described. Unconscious or mentally handicapped people from nursing homes voting. Entire project buildings voting for the same candidate. People voting 10 times. Etc.