The first thing one should understand is that no one in any of these types of positions should ever try to "train" or "manage" or "serve" a guest or guests until he has been truly trained himself in the field of etiquette, a lost tradition in our society today. Although it is claimed to be a "lost tradition," those guests being served in today's society can usually pick up on poor etiquette skills very quicklY, even if they do not practice these time-honored skills themselves. Because of this, the burden is always on the server, restaurant manager, or waitress.
For example, if your server has already served the main course to your party and notices one of the quests is missing, rather than asking questions, a "trained" server will simply walk over to the table to see if there was any indication at the table setting to let him know if he is returning. If he walks over and finds a napkin in the chair, the guest has simply excused himself for a moment and will return. If he walks over and finds a napkin set to the left of the plate with all of the eating utensils at 10:00 & 4:00, then the server knows this guest has completed his meal, and is ready for the server to remove his plates and bring his bill. If the party dining is not educated in these etiquette rules, then the server is put in the position of asking everyone questions, which is better than the dining party knowing more than the server! Etiquette training is vital in every business dealing with the public in any way as society is starting to appreciate respect a lot more these days and prefer to do business with people and spend their hard earned money with people who show them the kindness and respect once cherished among all of society quite easily.
There are many other things to come with proper etiquette training, such as: proper conversation, proper dress, no gum, not to be over-bearing, take care of your guests without irritating them, have a perfect understanding of the menu, and so much more. Etiquette does not stop at table manners!
I believe these examples give you an excellent overview of how a Hospitality Manager's knowledge of etiquette could definitely help a Manager be a good trainer from a guest's perspective, as well as from a server's perspective.