Your car's been sitting in the driveway for two weeks because you need a new alternator and don't want to pay the dealership. Or it won't start and there's no obvious way to get it to a shop. Mobile mechanics come to you - whether that's your home, your office, or the parking lot where you broke down.
A good mobile mechanic can handle 80% of what a shop can handle, at a lower price point (no shop overhead). Oil changes, brakes, battery, alternator, starter, sensors - all doable in a driveway or parking lot. They also tend to give more honest advice because they can't upsell you on other services they happen to have in stock.
Expect to pay 10-20% less than a traditional shop for most jobs. Ask upfront if the work they're recommending can actually be done mobile - some jobs genuinely need a lift. Verify they carry their own liability insurance. Red flag: anyone who won't give you a quote until they've already started the job.