Working with MIDI pads, controllers, and keyboards allows you to have a hands on feel when working with FL Studio and opens up many doors for the creative process. You may find that you have more creative control when turning physical knobs instead of a mouse or drawing in automation curves. It is extremely helpful when working with keyboards because you have 10 fingers instead of one mouse click. Working with pads can help when working with percussion samples and coming up with percussion grooves. You may also have faders/sliders that can control the faders/sliders (volume control) in the FL Studio mixer. In this manner, you can adjust the volume of multiple mixer tracks at the same time, similar to a standard analog mixer, but in this case you are using a MIDI controller to control software.
Many keyboards and MIDI controllers are also touch sensitive, which means the volume output will vary depending on how hard or how soft you press the keys...