It also serves to make the game more interesting. In Blarzmafia, people had basically figured to circle information about the mafia, but they somehow psyducked up and decided to vote against the wrong person. This led to the mafia going back and editing many previous posts and creating a rather elaborate cover up, which, though it never had and to come into play, was fairly interesting.
If people psyduck up, then it's not a big deal if they edit it. If they can do so before anyone notices, good for them. Though a lot if mods play mafia and they can see edits. Also PC has a view revisions option. I mean . . . if you're really that interested in what they've edited.
And, personally, from my experience as a GM, it's actually harder to keep track of votes when people make a new post and vote again, because I keep an eye out for the bolded text of a vote, and then also have to make sure that I'm not accidentally counting the same person voting against two different people. Especially since people tend to go against the recommended format of casting a vote (ex. using Kill, Burn, Execute, et cetera), which makes it so that the GM can't just use control F to locate the votes.
So far edits have proven no problem for mafia around here. I don't think it's a big deal. tbh I've never seen a no edit rule.