the bench is a great platform to gain muscle mass as it allows you to sit down in a strong position, push your feet against the floor have a long strong spine and the core is protected and strong, what that does is allow you to engage the muscles you want more fully in turn allowing you to grow muscle size and strength.

here are some exercises you can do for each body part using the bench

chest
bench press, incline press and decline press, as well as a narrow grip press, for muscle size i would leave flyes out of the equation
back, prone row, one arm row and reverse flyes
shoulders barbell military press, arnold press and lateral raises
biceps, curls, incline curls and alternating curls
triceps, one arm extension, french extension and skull crushers one of my favourite triceps exercises
quads, squats from the bench using dumbbells, barbells and single leg version, put the bench on the lowest decline and use it from there
hamstrings, leg curls
abs, aleknas from the bench

you could use the bench as a new way of working out, or you could use these exercises in addition to exercises that you already use

happy training and enjoy