Google will “allow” you a single custom URL of their choice. You cannot change it. You MUST give them an SMS number. Fuck this shit.
So, they're using SMS to authenticate you so some bad guy can't claim a URL with your name in it. They use SMS for 2 factor authentication in general - FWIW, I've never had Google SMSpam me. In fact, the only Google originated SMSs I have received are ones I initiated for 2 factor auth. That being said, they did just announce some new G+ features yesterday involving SMS. So, YMMV.