To go smoke some pot, while driving around. This is performed in situations where it isn't acceptable to smoke weed inside. This is applicable in situations where there is no important destination except to leave and smoke pot discreetly, as in not inside any building. Common: at home (to avoid family), around friends (who don't like that you smoke, or who want to mooch off your weed), or if you are working a job that isn't serious.
You're working somewhere (not that serious of job) and you have a co-worker friend, or a friend stops by, and they smoke the ganja. "Wanna go drive around?"
You live at home with your parents and you want to go smoke by yourself, or much preferably, with a friend. "I'm going to go drive around," you tell your parents, and it really is true. They just don't know you are going to hot-box the ride.
Any situation where smoking the herb must be done discreetly (as in most situations) so no other parties know. As long as you don't smell of it, hopefully; in most observed cases.