It all depends on personal preference and what type of lighting you plan to use on the tank really. Canopies allow you to conceal the top of the tank and match it with the stand, you can retrofit your lighting into the hood and conceal your equipment (ie: pipes, wires, etc). They also help keep the top of the tank out of reach from animals/people and also help in keeping fish from jumping out (provided the back is sealed as well).
There are other options though, such as satelite/orbit's which either hinge onto the sides of the tank or hang from the ceiling.