Question

IST is used for what purpose

Answer 1

According to latitudes on globe each point on globe has different time..
To set a correct time throughout  India we have assumed the latitude that is a standard time for whole of India. S we fallow IST that is INDIAN STANDARD TIME  

Answer 2

as there is a time lag of 2 hours from arunachalpradesh to gujarat. so,for having the same time through out the india  we use IST which is known as Indian Standard Time.