Uh, injectors flow fuel through them that can leave deposits behind that clog them. There is also the residual mist of fuel on the tips - pull out one of your injectors and take a look at the tips. I�ve had 24+ out of engines with less than 50K on them and they are dark brown. Then spray them with cleaner and the build up that was there is gone. It is clearly visible and that does not even address the build up that is INSIDE the injector.

Build up inside the injectors causes them to stick or not atomize the fuel - HD cleaners available at the dealer, lube shops and to some, over the counter take care of that. Fuel additives also will lessen that buildup but it not the same strength as the products that attach to the fuel rails and literally fuel the vehicle while they clean (w/ fuel pump disconnected).

For more info, see:
http://auto.howstuffworks.com/framed...s/finject1.htm