While discrete hand signals may not mean much to a middle of the road dining establishment you can bet that the people who implement these protocols do so because they are effective.
They essentially communicate a level of professional behavior when working around their patrons. Small things like this are what distinguishes an apprentice from an artisan.
Agreed, I've noticed this type of signaling often in the hospitality industry. I'd compare it to a DSL with which a leader can communicate unobtrusively with those they lead. In certain cases it takes more time and effort to communicate information or instructions verbally, and it's more noticeable.
When I was a teenager, I delivered furniture with my dad, who was a furniture refinisher. He used to pride himself on being able to deliver furniture without ever saying a word between himself and his delivery partner, and trained me to do the same. The reason, he said, was that it shows a level of professionalism to be able to competently make the delivery, navigate the halls without scratching anything, set everything up, and never talk at all. Certainly it made the job more fun to do.