Many websites asks you for e-mail address to download software or to register. But you don't know what they'll be doing with your e-mail..
Your e-mail will probably be stored in a database which may be get to the hands of spammers and you will be receiving unsolicited bulk e-mail.
http://www.spambox.us/ can create a temporary e-mail address for you that will expire in the time you chose, all the mails directed to this e-mail will be transparently forwarded to your real e-mail.
For example, you need to register on a forum that requires e-mail verification, so you generate a spambox e-mail with an expiry of one hour.
The generated e-mail will expire in an hour, and this time is enough for the forum to send you verification code.