I understand that users with less than 50 reputation can't comment, but sometimes answers from such low-reputation users get converted to comments by moderators, so it's possible that such comments would come to be.
Now, these users can edit their own answers, but not their own comments. Why?
Perhaps some explanation is in order (my apologies for this being a personal story, but it demonstrates the point marvelously):
I recently found an answered question whose answer seemed out of date, or was inaccurate for Windows 10. The answer itself is excellent, but it points to the wrong file - either the file was changed in some update since then or it's different on Windows 10. At any rate, I found the file myself and wanted to add a comment to that answer with this information.
I don't use StackExchange to ask or answer questions though and therefore I don't have enough reputation to add a comment, so for lack of choice I added this information as its own answer. A moderator later turned this to a comment, which was great.
A few days later I noticed that in a stroke of brilliance I only wrote the path to the file's directory and not the name of the file itself. So I wanted to edit my post-turned-comment to add this information, but I couldn't. I thought to ask the moderator who made the conversion, but there's no messaging system. Devoid of any alternative, I once again had resort to adding this information in an answer (editing my original, converted answer didn't update the comment). In my answer I added a note for moderators shortly describing the situation and asking for this information to be added to my comment if possible. It seems like a terrible form of communication to me, but again - I didn't seem to have any other choice if I wanted to add this useful information.
My request was granted, but with it came a warning that if I continued posting answers instead of comments I would be banned from posting answers. But what was I supposed to do, then?
Another user commented on my second answer saying I should ask a good question or give a good answer to get the reputation points I need, but I had no desire to go looking for questions I can answer just so I could edit my own comment for the sake of saving some other people time and effort, and I certainly wasn't going to make up a question when I had none to ask.
I get the minimum reputation restriction. But once a user has a comment on an answer or a question already, why prevent him/her from editing it? Or at the very least add another comment on a post s/he's already commented on? I felt like I was treated as a troll for adding useful information in the only way I was able to.