India is a large contry. it covers a long range of longitude. as we know that variation in time is caused by longitudinal difference,so the time difference between the westernmost and the easternmost longitudes of india should be very large. to avoid this, we designate the 82*30`E longitude as standard meridian.
The time at this longitude is accepted all across india as Indian Standard there is no such problem of time in the whole country due to standard meridian.