Well a bench/barbell can be used to develop a lot of different muscles besides your chest, but I'm guessing you are specifically referring to the actual bench press motion. If it is a machine you are buying then the majority of the work will be done by your pectorals (chest) -- lower pectorals with a flat bench press --upper pectorals with an incline and your triceps will receive a bit of benefit also.
Free weights, on the other hand, result in the development of smaller stabilizer muscles that help to stabilize the weight as you lift it. Just think about a machine, you either push it up or resist the bar as it comes down - there is only one way it can go. Lifting a free weight, though, the bar could go anywhere besides straight up and down, therefore smaller muscles you don't regularly target up towards your shoulders and down towards your lats also get worked. In addition to all that, there are different parts of your chest (upper and lower pectorals that can be developed using variations of the bench press.
Specifically the muscles that the bench press motion will benefit the most are the pectorals (upper and lower), triceps, and front deltoids (shoulders). The beauty about a bench and barbell is that you can use it to develop pretty much any muscle you want. Did that clarify things a bit?