Giving a treat following a behavior or series of behaviors, the dog may be unsure what exactly
the reward was for...unless its Lassie or another dog who understands English.
Think of it this way.
If you were teaching someone a series of behaviors, like martial arts,
it would look like this: "Show me a front kick" (command), "yes that
was the correct move"(click), "good job" (reward)." The "click" does that for you. It also allows you to chain small behaviors together to form a more complex behavior.
For example. Front snap kick. Bring leg up (click)...Bring leg up
(click)...Bring leg up (no click)..bring leg up..extend leg (click).
Think of the click as a way of explaining to an animal "that is what I
want." Sure, the treat relays the same message but the click just makes it more percise. Its how trainers get dolphins to do flips and spins on command