The purposes of a canopy are to mainly hang DIY lights, and lights that don't look good without being covered. The purpose is also to eliminate light splashing out of the tank. In the old days canopies were painted on the inside to reflect as much light into the tank as possible. These days with the development of aquarium specific light fixtures, it's easy to just buy a fixture, and sometimes it even looks better.
IF you are worried about your fish jumping, what most people do is buy something called Egg Crate - it's a light diffuser for florescent light fixtures. You take a side cutter and cut it out to fit the top of your aquarium. I personally prefer to paint my egg crate black with rustoleum flat black paint.
Egg crate is cheap, and it also is useful for other applications like frag racks, building structures, filtration systems, HOB filter mods... you name it.
