What are the benefits of locked iPhone over an unlocked one?

I'm planning to buy an iPhone, I would like to know what the main difference between a locked and unlocked iPhone is. Plus, what advantages or disadvantages do these 2 different modes have?

If I buy a locked iPhone, will I be able to use the same Airtel SIM I'm using now?

It just means you won't be able to use your Airtel SIM if you buy it locked. So buy it unlocked. Otherwise there's no difference.

An unlocked one could be used on any GSM carrier in the world. A locked one is only limited to one.

No advantages or disadvantages.

If you bought a locked one it would work on that network only if it was locked to it in the first place.