Everyone knows by now that Cell Phone Network Providers don't want you to switch to another provider.
To prevent that, they lock your phone, meaning, when you insert another carriers sim card, your phone will ask for a
SIM NETWORK UNLOCK PIN or simply say "sim card not supported". It's just a bad tactic they use as part of their customer retention process.
So no matter how happy your are with your current provider, there's always a good reason to have your phone unlocked.