Sometimes when I edit my messages, you get a thing that says that "message was edited by InvivnI" on some of my posts, and other times, the "edit by" legend does not show up? How does this happen?

If you edit your post within 5 minutes of posting it, then the tag will not appear.

Actually, it was recently changed to 10 minutes.

quote Final Blade

_{Why do we have the tags in the first place? It makes the posts dull and unclean imo.
Times where I make a good looking posts, gets ruined by the edit thing.}

Moderators can remove them if the post is a sticky or something, but it's mostly there to preserve any evidence that a post was edited in the event of an issue.

Even if the tags are removed, they can still be seen by highlighting the area they would normally appear in.

BTW, regular members can do this to.
Put this at the very end of your edited post:
Light background~[color=#EEF0F9]
Dark background~[color=#C6D0E7]

That will hide the edit tag. If you edit multiple times, be sure to remove the previous edit tags so your post doesn't stretch.

Also, if you download NAMFox for the Firefox web browser, then it will do that automatically with a little tick box below the edit screen.