What is the role of concentrated sulfuric acid in the nitration of toluene?

Answer 1

It's a catalyst. It protonates the nitric acid, in order (eventually) to make the (NO2)+ ion, and then is recreated at the end of the reaction, by removing a proton from the benzene ring.

Answer 2

The concentrated sulfuric acid functions as a catalyst and at the same time a dehydrating agent that absorbs water to drive the reaction forward. It will also aid in the formation of nitronium ion (NO2+).