I think mostly it’s because people change their minds about what they’ve posted, or as @Deanna32 says, posted in the wrong thread. If you “mess up” your post you can just edit it to make changes rather than deleting it and starting over.
Deleted posts become ‘withdrawn’ and will disappear from view after 24 hours but you can change your mind too, and unbin it in a similar way within that 24 hours.
What were you trying to copy and paste? For just trying out forum features or playing around making pictures into links or whatever, The Sandbox is a great place to play. There’s lots of information in it too about things you can do as it’s where a lot of us tried stuff out when this incarnation of the Forum first launched.