There's value in editing for clarity within a window of a live discussion. After the live discussion is less active, it's important to be able to reference things or see a coherent view of the discussion and what people were responding to.

Yes, it's because the comments create a discussion thread that then becomes impossible to follow (or worse, misleading) if certain comments within it are either deleted or edited to say something different. The idea is that what you write becomes communal property once it's been responded to, because it's part of a community discussion that loses meaning if people start deleting individual comments.