What do you mean by "need"?
All plants need co2 to live, land based ones get it from the air, water plants get it from the water, plants absorb co2 & give out oxygen. co2 usualy comes from the fish themselfs, like all animals they breath in oxygen & breath out co2. An injected co2 system basicly does exactly what is says on the tin. It introduces an artificialy high level of co2 into the aquarium which in turn encourages the plants to grow.
A diffuser is a device that allows the co2 to be absorbed by the water colomn. So if you are running an injected co2 system then you do need a diffuser.
To summise. co2 is not a requirement in the fishkeeping hobby but it will make your plants grow faster & healthier.
Tom
