Its not international standard time, its indian standard time. (ist) we have it since india is a vast country, and it needs uniformity in time. if there will be different times at different places, then there would be a lot of confusion. hence, to avoid this confusion, median 82'30 degrees passing near allahabad is taken as the standard time throughout the country.
if we follow GMT (+5:30hrs) i.e. greenwich mean time, then too it would be a lot confusing because of the different positions of the sun at different times. so therefore india has adopted its own standard time. IST is followed throughout India, and in Sri Lanka.
hope you find it helpful. happy to help. :))