Last updated 06-APR-11.
There are many benefits to unlocking your phone.
The most obvious benefit is that you can switch network and usually get a better deal elsewhere. Many sites track the best mobile offers out there. Finding them is just a google-search away.
Ever wanted to use your phone abroad and put a SIM from a local company in it to save on roaming rates? In most cases you will need to have an unlocked phone (though having one is no guarantee that your phone will work abroad A? it will need to be compatible with the local frequency. Find out more on frequencies here
Unlocked handsets are worth more in the market which means you can sell them for more money. Go on ebay for evidence of this.
Even if they don't want to switch from their current network, many people want to use different SIMs in the same phone. There are a number of reasons why you might want to do this, here are some: your friend has run out of battery and needs to use her SIM using your phone; you discover it is cheaper to use the internet/call international numbers on your mobile when using a different SIM; you lead a double-life which you like to keep separate by using different numbers (we're not here to judge)