No more roaming charges
You will be able to switch simcards when you travel, and use the simcard of a local service provider, which will allow you to avoid the high roaming charges that your service provider back home would charge you.
Switch freely between service providers
If you decide to switch service provider, you can keep your phone and use it with your new service provider (as long as you switch over to a GSM provider).
Increase resale value of your phone
Since an unlocked phone can work with any GSM service provider, you have a much larger target audience to whom you can sell the phone to. This also means that you can sell it for more.
Buy any phone you want
The beauty of being able to unlock a phone is that you are no longer restricted to having to buy one of the phones that your servide provider sells. You can go ahead and buy any GSM cellphone you want, unlock it, and use it with your service provider. Gone are the days where you have to hope that your servide provider has the phone you’ve been wanting. You can just go out and buy it, unlock it, and use it!