Original: The other way that trading is made instrumental in gameplay is that many PokÃ mon, like Haunter and Machoke, can not evolve into their final forms without being traded. Gold and Silver built upon this idea with the advent of holding items; some PokÃ mon only evolve when given certain items to be held and then trading them.
Simplified: Some PokÃ mon only evolve if they are traded to a friend.