Editing serves for quick corrections.
The goal of the forum is to gather knowledge. We can learn by reading archived discussions of others. (This is big difference compared to chat).
More members usually cooperates on a topic. If you delete bits of discussion it will become unreadable (and it makes posts of other members incomprehensible).
Another reason: This forum is archived using external online archiving services. If you post here it will become part of the public internet even if you delete it later.
If you just want quick answer to your question without having it archived indefinitely, use our chat.