When you buy a Blackberry mobile device you buy it from a mobile service provider.
The device will be branded and locked to the carrier you got your Blackberry from.
Blackberry which is locked to a carrier comes with a secured setting that can be unlocked using a unique unlock code.
Without the unlocking the phone is locked by a SIM lock to the branding carrier and can't be used with other carrier.
After using the unlock code and unlocking your blackberry you will have a SIM free Blackberry device which is no longer locked to any carrier (the sim lock is disabled by the unlock code) and then the device can be used with any other mobile carrier operator that operates over the GSM network.
without unlocking you can't use sim cards from other carrier because the sim lock will not allow signal and connection. you will get SOS screen.
the benefits are, well - first of all there is no need to unlock it each time, once a blackberry phone is unlocked it's unlocked for ever (even if wiped) and you can swap unlimited sim cards and carrier.
This is good you you wont to use other carrier without buying a new device / keeping your old device.. or if you want to use a few lines / carrier... you can use prepaid sim cards as well..
If you want to sell your Blackberry it will be a lot more easy to sell unlocked blackberry phone because you will reach a lot more clients, any one that uses a gsm sim card can use unlocked blackberry and anyone can get a cheap GSM sim card anywhere.
and it's also very good solution for traveling if you want to save some money by using your unlocked blackberry with pre-paid / contract local sim cards.