Re: bench press
Bench will help improve strength in the chest (obviously) and also arms, triceps mostly. Your back and shoulders will also strengthen as you use them to control the weight.
Bench is a good excersize if you want to incorporate weight into your routine. Just like clean and jerks are and military press. You don't need to though unless you want to bulk up. The strength you get from boxing excersizes, when done correctly, should give you the strength you need for the sport.
Oh and if you weight train, I think the best to do are squats and deadlifts, next being back and neck excersizes, and lastly chest/arms.