The list of repairs that can be done on site is longer than most people expect: diagnostics, brakes, batteries, alternators, starters, oil changes, belts, hoses, thermostats, water pumps, spark plugs, ignition coils, sensors, many gaskets, AC service, and a wide range of electrical work. That covers the large majority of what a general repair shop sees in a week.
What still needs a shop: anything requiring a machine shop, transmission rebuilds, most exhaust welding, engine removal, and work that can only be done safely with the car on a lift for hours. A good mobile mechanic will tell you this up front instead of attempting it in your driveway.
On cost, mobile service usually lands at or below shop pricing. There's no tow, no shop overhead built into the labor rate, and you're not paying for a waiting room. The bigger savings is time: no drop-off, no ride home, no second trip to pick up the car.
The best use of a mobile mechanic is diagnosis first. If the car has a problem and you're not sure how serious it is, a mobile diagnostic gives you an answer at your driveway, and in most cases the repair happens right there too.