Just guessing but id say it reduces the chance of detection. being quiet as a mouse in sneak.. less chance of getting noticed???
On the subject of noise levels, could the footstep sound effect be turned up, the only way to hear it is to turn your PC up pretty loud which isn't adviseable when you could be shot at " width="15" height="15" /> There is nothing so simple that can absorb you into a game than hearing the footsteps as the characters walk.
That would be the most annoying sound ever!
Never ever turn that up!
When you shoot an NPC, an NPC nearby will sometimes take notice and shoot at you. When using a silenced weapon, this, and Agent spawns, will happen less often. This is where the "noise level" comes in.