Why unlock your phone?
When you buy a new phone, chances are it'll be locked to the network you bought it on - and if you try to put another network's Sim card in, it simply won't work. Unlocking it means you can use the phone with any compatible Sim card, on any network. This has several benefits:
Grab promotional deals Often networks give away free Sim cards, which can come with free texts or calls. Yet if your phone isn't unlocked, you can't put them in and take advantage.
Freedom to roamUnlocking means you can use foreign Sim cards to cut phone costs while you're abroad.
Added handset value Unlocked phones tend to sell for more on websites such as eBay, because they have much wider appeal to users on other networks and in other countries.
In short, unlocking gives your mobile independence from the network you got it from - and it's usually simple to do. Many older handsets can be unlocked using codes generated free on the web, and if that doesn't work, you can do it for a reasonable fee, either through your network, or high street phone unlockers.