Dear Padmapriya Janakiraman,

Drying under nitrogen stream is to avoid any oxidation of the compounds. Nitrogen removes oxygen (air) which is the source of the chemicals oxidation. 

It is necessary to dry under nitrogen stream, especially for natural products which are suspicious to oxidation by air or oxygen.

Hoping this will be helpful,

Rafik

Dear Rafik Karaman,

Yes it is clear. Thank you sir

Yes, Dr Rafik has answered it well. Being inert gas, it eliminates the possibility of any other reaction.