Why do CIA and other spy types touch their ears in movies? And do people really do that?

I thought it was just a cliche, a convention in the movies, because if they’re receiving a message, it should just come through their earpiece without their having to touch it, right?

So why do they have to touch it?

In the novel I’m reading, a bad guy is spotted touching his ear like that. So I take it that they really do that. Why?

I tried to Google this but couldn’t come up with the right search terms.