A typical San Diego house has a furnace and an air conditioner. Two machines, two seasons, and one set of ducts, one blower, one filter, and one thermostat between them.
That shared middle is where problems get missed. A restricted return makes the furnace cycle on its limit switch in January and makes the air conditioner ice its coil in July. Serviced separately, that gets diagnosed twice and understood once. Serviced together, the airflow gets measured once and the answer applies to both.
The other reason is timing. The changeover between seasons is a natural moment to check both, and it means you are not booking a visit during the week when everybody else has just turned their heat on.