Are you planning on putting a new aftermarket exhaust on your Suzuki DR-Z400? Or maybe you had the header removed too many times, and it might be leaking at the cylinder head now?
 
Anyways, this gasket seals the header pipe to the cylinder head by being crushed during installation.
 
It's not a one-time-use type of gasket, but after a few times, it just won't have the ability to conform to the shape and seal properly. It's a good idea to get a fresh one when installing a new exhaust system or restore an old bushpig to avoid having an exhaust leak.
 
Note: An exhaust leak can affect the overall engine performance with loss of acceleration and power. It is usually detected if making extra noises or seeing traces of burnt carbons around the port.
 
This is a Suzuki Genuine Parts:
14181-29E02-000 GASKET, EXHAUST PIPE