Why do I need to use this email address

What email address?

The @icloud.com version of your address is an addition to the @me.com version which delivers into the main inbox (it isn't a separate account). It's been introduced to reflect the branding of the service, but people like yourself who already have an @me.com address can continue to use it as before. There is no obligation to use the @icloud.com address if you don't want to. However any aliases you create in future will be @icloud.com only.

Because all the cool kids are. Don't you want to be cool, too?