What does it do for me if I use the equipment in the gym that work on my back, and back area?

So like... Will it make my shoulders bigger, and back bigger? I don't understand, I am fourteen and want to be a powerful sprinter, but also want to have large muscles and nice pecs, so... Not sure what the back machines do? Like the pulldown, and etc Basically the equipment that works on your back area and shoulders, thankyou xx