That engages the rollers, and controlls the delay of cycling. Due to teh increased backpressure of a suppressor, it would allow the bolt to unlock with less stress tahn a regular peice.
If you don't have a suppressor, you don't really need it.
ETA: Do you have an HK94 clone, or the SP89 one?
Is it a #28 peice in teh bag, or just looks like that?