The advantage of single time zone in India is:
You don't need constantly to adjust your time in watches and mobile phones when you are travelling across the country.
It makes easy to compute time.
This also ensures that whole country is functioning at same time.
For Example, A flight from Ahmedabad to Guwahati takes 2 hours. If you start at 6 pm ,you will reach by 8 pm. But according to time zone Ahmaedabad and Guwahati would have 2 hrs difference in them. So according to their time zone you would have reach by 10pm making it completely confusing. So a single time zone is very good for our country.