Whenever you sign up for a new service, they usually ask for your email address. This is usually a problem if you hate getting spam. One solutions is to use disposable email addresses. Gmail has one way to do this using the + sign at the end (e.g. if your email is firstname.lastname@example.org, any mail sent to email@example.com will be sent to you as well), however this has a few problems. The first problem being that someone can easily figure out your real email address, by removing everything after the plus sign. The second problem is that many websites don't let you use the plus sign in your email address.
This is where Yahoo mail comes in. You will need a premium account, however if you are an AT&T customer (as most iPhone owners are in the US), you can get one for free.