Say you're at a conference, away from your usual ISP, and you're trying to send email, but all port 25 traffic is blocked. Say also you have .mac, for which you keep paying and paying and paying to have a reliable external email address, and sending email is also blocked. .mac (and perhaps other places) use an alternate port (port 587 in .mac's case) which can be use to work around this. Now I can send email, and life is happy.