I’ve put my business cards in quite a few fish-bowl drawings, because the amount of personal information I’ll give away for a free chili-cheese burrito is astounding.
At some point, the proprietor of such a card-collecting eatery might pay her angsty nephew to hand jam those email addresses into a spreadsheet. Odds are that one of those addresses is going into that list with a typo. The same thing happens with a single opt-in webform (huzzah for double opt-in).
You might think most of these typo addresses are going to bounce when you send to them, so no big deal—typos are merely a minor annoyance and occasional source of hilariousness. And when they bounce, you’ll just clean them up then.