Is it necessary to store purchased sealed Coffee powder in freezer? Why?
It is NOT necessary to store coffee in the freezer. 

It can be helpful if you want to store it for long periods - i.e. months. 

Ground coffee should NOT be thawed and re-frozen, nor opened frequently and put-back in the freezer. Think of your freezer as long-term storage, and take out a week's worth of coffee at a time. 

Also, it's not going to make a huge difference either way - if you want better tasting coffee the best bang-for-buck would be buying whole-bean coffee and grinding it as you need it. 

Here's an obsessively detailed taste-testing account:

http://www.home-barista.com/store-coffee-in-freezer-conclusions.html