A delete button.
With this button you can delete your own comment... any more I need to say?

No... because what if you posted... but after posting you find no use in that post... and you want to delete.
Or you post in the wrong section.
Or if you post at the same time as someone else and you say the same thing as the other guy.
Or if you post something and find out it's already been said.
In all of those situations you won't know what to say if you edit.
Instead you could just delete your post.