What’s the benefit of doing an M.Tech in mechanical engineering from an IIT if you already have a placement offer from a decent company?

The kind of exposure to mechanical engineering which you would never get from your undergraduate course in a smaller institute. After B.Tech, we hardly get an introduction to career options other than the regular ‘placement’ offers. But an M Tech would better clarify the kind of career you want to pursue – industry, academia whatever… and yes, the placements are good too.