When you are going to invest your money into something as expensive as a new guitar, it helps for the clerk to be knowledgeable on all of their products. If the clerk can’t answer any of your questions and has no idea what you are talking about, you are going to have a bad experience and probably not want to even buy your new guitar from that specific shop.
It is a big investment to buy a brand new guitar and if you are not able to get all of the information that you need on what you are interested in, you could end up purchasing a guitar that you will end up not wanting because it is not the right one for you.
It is almost like buying a new car. You would not want to buy a brand new car that you have no information on because the clerk could not answer your questions and had no idea what you were talking about.