Wherever I was in the world (aside from North Korea), I had a local cell phone number. They're usually pretty easy to get, so long as you have an unlocked GSM phone. But I wanted to make sure that there was a single number where my friends and family could contact me. I wanted to make sure that I didn't lose my old cell phone number in the US, either.
So, I ported my cell number from T-Mobile to Callcentric. They have a lot of really cool features, like being able to ring three other phones simultaneously when someone calls your Callcentric number. Plus, they're a SIP VoIP (voice-over-IP) provider, which means that you can make cheap Internet phone calls from your laptop or SIP-ready mobile phone, wherever you are in the world. I've written up some notes on getting it working with my Nokia E5-00 here.
What) this meant was that my friends or parents could reach me by dialing one phone number in the US, wherever I was in the world. That call would forward to whatever local number I had. Of course, I could also set rules so that my phone wouldn't ring in the middle of the night :). And it's cheap! I've set up another account with Callcentric to replace my parents' old landline for $3 a month.
There's also lots of essential services that require a mailing address. You might be backpacking through Australia, but having someone checking your mail back in America can be invaluable. Or you might want to get things shipped to your address in the US, and then forwarded to you somewhere else.
That's why I use services from Mail Link (in the US) and Private Box (in New Zealand). They'll accept any letter or package, and tell you who sent it to you. If it sounds interesting, they'll scan your mail and send it on to you via email. They'll also forward mail to a destination of your choosing.
Even if I settle down with my own home in the future, I'm still planning on using these services to help preserve my privacy and cut down on junk mail.