Maybe they are thinking of being more inclusive by covering campervans which don’t have always have showers and toilets but saying “Motorhomes” as a catch all.
Also the loos and showers will likely also cover other marina users that they are trying to encourage to visit - boat users, day trippers etc or those doing water sports, cycling maybe. If they are putting them in anyway for a wider user base you would be better looking at them than looking for them?
But yes not all of this is needed if towns and villages want to provide a simple cheap overnight stop with no facilities and avoiding a lot of planning permission hassles - consent to discharge etc for toilets and showers. As long as they know they can make it as basic as they like. It could be as cheap for them as an angle grinder blade to cut down the no overnight parking signs