Used as an affirmation of a point made by a fellow interlocutor — a point of view that you have always supported and that the other person has just come to support. It is mostly used in a congratulatory, if slightly patronizing, manner.
Subu: Macha, I'm calling Airtel tomorrow and asking them to deactivate my 3G. I'm switching back to GPRS.
Pooty: But why? Isn't 3G fast! You can even stream Why this Kolaveri Di on your Android in BMTC bus stand while waiting for bus ... you are doing this because it's too expensive, no?
Subu: Yes, with that money I can go to bar every weekend.
Pooty: That's what ...