They're part of the drivetrain that propels the bike forward.
Don't know what you mean by silent chain.
They're used for power transfer and to create gearing ratios.
Depends on the specific design of the bike. see links below for more.
To provide power from the motor to the rear wheel in which to make it go
Sprocket and chain and what?
How to Clean and Lube a Bicycle Chain
Operation of a Chain Saw
Chain Saw Maintenance and Storage
Starting a Gas Chain Saw