Here is how it works:
When the caller dials your number or extension, instead of hearing music while the call is being transferred to you, the caller hears ringing. If you take the call, then you have the caller live. If not, the caller is routed to voicemail

What's the benefit?
Think about it, when you call a company and you hear ringing as opposed to Hold music, you feel like you are about to get through to someone. There is something mentally gratifying to the caller about hearing ringing, especially if they just navigated through an auto attendant menu.

It will enhance the callers perception of your accessibility.

This also serves to reduce the perception that the call is being handled by an automated system.