It's mostly the training methods based on the principle of operant conditioning which have the most effect. The clicker is great to get an even more precise marker of a particular behavior. Some people use a short, quick word. If I don't have my clicker on me, I'll use, "yes!" And then praise/treat. It's just that the sound of the clicker is very distinct and really differs from our voices which they hear all day long. So, as a communication or as a way to mark a behavior out to the dog, the clicker is very clear. You can do both.
Of course, hand signals are great and IMO necessary also. And they're easy because 9 times out of 10, you're using them without realizing it when you show the dog or lure the dog with a treat and that just becomes the hand signal automatically. Like with sit or down. I just invented a new one for Lyric with his new trick to bow. I swing my arm down like I'm bowing...LOL and have my palm facing up. I'll get it more subtle later.