Cuz if you do and make ur pokemon hold it and battle with that same pokemon the
exp points double if not they increase.

My sources: experience

U need it for pokemon to gain exp without battiling

If you beat the whole game, go to fushia city and go to the eastern exit and have one of
your pokemon hold a lucky egg (item)
And walk, run or bike in the tall grass and battle a chancey. The leader at the front
should have a lucky egg, beat chancey and get a load of experience to your pokemon.