The reason a crib is useful is because the enigma can't route a letter back to itself. So, you can slide the crib along the message until no letters line up, and that's possibly where it is. If your crib is "the", that's not terribly useful, because it could exist anywhere. The longer the better.

I ONLY know this because https://www.youtube.com/watch?v=JsBZOcqZerk, btw.