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Say you have a domain and you want to give out free e-mail addresses to anyone who registers on your site. Manually doing this would take forever. The webhost we use wrote their own webmail software. To sign up a new account all I have to do is pass in an admins username and password, the new users desired email address, and how many megs of space to give them. The service then sends me a return code about half a second later telling me the e-mail account was created.
Other examples include online credit card processing.
