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1. Prevents typos (though bounced emails are not a big deal)

2. Makes it so you can't sign someone else up surreptitiously

3. Ensures the email they enter is real

Depending on the site, these may or may not be a concern. For some sites, the drop-off rate is worth the reduction in support hassle.

If I may, I'd like to repeat and emphasize, these two points are very big, in my experience...

2. Makes it so you can't sign someone else up surreptitiously

For some sites, the drop-off rate is worth the reduction in support hassle.