I think at one point I had about 35 of my own maps/attempts and it became a real pain to figure out what was what. 2 seconds taking an in level screenshot and arrowing down to, "add as map title screenshot" would have saved me hours of searching by the time I stopped playing Reach multiplayer. I can't remember how many Forge user creations I downloaded over the years. I had to create a rule to delete maps immediately if we weren't enamored with a map, otherwise it just became another map in a massive list that I couldn't remember.
Screenshots make the maps stand out in the list, which in turn speeds up the time it takes to set up a playlist. The fact that this is the easiest feature to implement just makes it even more frustrating when it's not there.