There are places in the game where important people/Pokemon have stood at one point. If you use the Itemfinder on the EXACT spot where they were, a special item will be found which is usually very rare. You cannot find these items any other way.

Item	How to unlock
Leftovers	Use Itemfinder where the 2 Snorlaxes appeared.
Macho Brace	Use Itemfinder where Giovanni was in Viridian Gym.
PP Max	Use Itemfinder on the single ground tile to the right of the waterfall on Two Island.
Soothe Bell	Use Itemfinder where Mr. Fuji was stinding in Pokemon Tower.