Well thats a nice question Discover 26,
See the base year is always in which there was minimum fluctuation in prices. Let's take the case of CPI, in this index, the base year used to be 1995-96 for quite some time but recently the base year was changed to 2005-06. The reason for such a change is to reflect the true picture of cost of living in a nation. For example - Suppose the price of sugar was Rs.10/Kg in 1995-96, which rose to Rs.25/Kg in 2005-06 and presently, it is around Rs.50/Kg. Thus, the increase in CPI (not inflation) will be 100% in comparison to 2005-06 and 400% in comparison to 1995-96.
We cannot go on comparing current prices with the prices which used to be 30 years ago. Also, there is a concept of "Basket of goods", which remains constant in CPI but the consumption habit of consumers keeps on changing every day. So, to capture the real consumption habits of consumers, it is important to revise the basket of goods after a few years. This also necessitates change in the base year (period).