By un-submit, I mean - prior to a meme being accepted/approved.
There are many reasons a person might want to un-submit a meme. Maybe you realized that there was a typo, or incorrectly presented information - or even something you feel you could have worded better.
Deleting the image doesn't recover your submission and allow you to submit another - you get as many as you get in a stream, and once you burn one of those submissions - it's burned, regardless of if you allow it to make it to approval or not.
Additionally, some mods delay approval for some memes - which causes them to appear further down the list than memes that are approved expediently. Unsubmitting a meme might allow a creator to reset their place in the queue - negating the efforts of a malicious mod who is delaying a meme to reduce its reach.