Do dishes used for preparing food need to be washed with soap? It seems to me that washing dishes with soap serves two purposes:


To remove food residue from the dishes
To kill bacteria that got on the dishes


As far as I know, the primary way bacteria get on dishes is from coming in contact with human saliva. But that wouldn't happen with dishes that are used exclusively for preparing food and not for eating it. So, assuming good residue can be removed without soap, is there any benefit to washing dishes used for preparing food with soap?

For the purposes of this question, assume we are talking about a home kitchen and not a restaurant.
Human saliva is not your only concern.  There are a variety of pathogens that could be problematic if food preparation items (hands, utensils, cutting boards, counter tops, and containers of all sorts) are not properly sanitized. This includes potential pathogens that already exist on the raw food itself. Soap is not necessarily required, but some type of sanitizer is.  Dish soap just happens to also remove grease, and is most commonly found in the home kitchen.