There are many mobile phones are "locked" into one particular service provider once purchased it. A user cannot use the handset to switch into another service provider by replacing available SIM card module into it.
So the best reason is to unlock the phone so it will be used into another network which is very useful when traveling from country to country.
Unlocking is completely legal and not illegal, because every phone user own the phone after all!
the only reasons that the networks do it is to try to make sure that the phone users don't move to another network, but all it really does is stop them using the phone as they would like to!
But how to Unlock a Phone?
There are two methods in unlocking mobile phones, It varies a certain products of mobile phones some can be easily unlock and some are not.
1. Unlocking by entering an unlock code. Many Nokia, Samsung, Siemens, Sony, Panasonic and some other brands of mobile phone will unlock if you enter a specific code based on your phone's IMEI (serial) number. This is the most easy method of unlocking - all you have to do is get hold of the code!
There are some that offers free unlock codes and some you need to purchased it via online.
2. If a model of phone does not support unlocking by code, you need to get the phone unlocked using a unlocking device and a unlocking software tool.