I believe the theory is if you make users type it twice you can spot if they're likely to get it wrong and force them to get it right - for many signups it can

I believe the theory is if you make users type it twice you can spot if they're likely to get it wrong and force them to get it right - for many signups it can be a vital piece of information, after all. Since email addresses are generally autocompleted by my browser, I can't remember not simply copying/pasting between the fields. I like the idea of verifying the password, this is how password programs usually work.