It's goes with the terrain. At dive stores, instructors do everything from cleaning the heads, rinsing the rental gear, fixing equipment, filling the tanks.... and rotate as retail staff to attend people who come in the store looking for courses, dives and/or equipment.To answer the main question, why do instructors have to sell anything?
A lot of divers find it reassuring to buy equipment from an instructor, especially if they have trained or dived with that instructor who will usually either know how much and what kind of diving the customer does or will ask relevant questions.
At large specialist dive equipment stores, most of the staff are either instructors or DMs and that is one of the qualifications the store insists on to hire somebody. It makes sense for a lot of divers who live and work in cities because they will have there regs and other pieces of equipment serviced there, often by the same person who sold them the equipment in the first place. The store becomes a social gathering point where people can stop by on their way home after work, ask questions they have about equipment they are thinking of buying, chat about diving and so on. Many stores like this organise trips for their patrons.
Manufacturers want instructors to sell their equipment and often organise seminars and visits to the stores to familiarise instructors with equipment and answer any questions about it. Having a relevant instructor-level qualification is often a prerequisite to work as a salesperson for a manufacturer.