They lessen power and accuracy, but if you get a good quality supressor, they actually make sound from your replica quieter.

 

Suppressors are useless for noise reduction on AEG's as the loudest sound is the gearbox. However, they can be benificial on a Gas Gun, if it's a decent suppressor. The main pourpose is to protect extended barrells. For example, you can put a 28in tightbore on an M16 if you have a suppressor.