Why do I need to change my rocker head gasket?

My bike is Suzuki GSX600F and my mechanic told me that I need to replace the Rocker Head Gasket. What's the function of that particular thing? Is it gonna effect the performance of the bike?Why do I need to change my rocker head gasket?well the head gasket is the gasket between the engine block and the head. that gasket holds in all the compression from the explosions. it there is a leak, you will loose alot of power.