If you have a foreign car, you may not think that a specialized mechanic, or one who works on that type of car specifically, is needed, and you may assume that all mechanics are alike. It's true that the vast majority of mechanics can perform routine service on just about every make and model of car, but for many repairs, it's good to choose someone who specializes in your foreign car brand in particular. Note why that is, so you can ensure your car gets the service it needs and is always in good repair.
Not all domestic parts will fit a foreign car precisely; even everyday parts that need consistent replacing, such as a battery and hoses, can be a poor fit for a foreign car. A mechanic may try to force such a fit, or it may be that a poor-fitting part begins to rattle and come loose, or causes other parts to come loose as well. This can lead to poor performance from that part and your car in general; you may then face additional repairs, or early replacement for those parts, simply because they didn't fit right and work properly. A specialized mechanic will have access to the parts you need, and will know how to fit them properly.
Specialized mechanics will usually be more familiar with your make and model of a car, and with how to diagnose certain repairs. They may know what certain noises mean, or what parts may be failing when the car stalls or otherwise acts up. They may also have specialized equipment that can easily pinpoint those problems; this can mean faster repairs, as the mechanic won't need to spend as much time simply diagnosing your car's problems before those repairs are performed.
A specialized mechanic will know what your car needs by way of regular maintenance, based on common issues with that particular make and model. For example, some particular cars may tend to run very hot, and others may tend to have brakes that wear out quickly. If you know that your particular make and model of car will experience certain problems or mechanical issues, you can then have maintenance performed as needed; this might include checking the coolant levels before they get too low and the engine overheats, or changing out the brake pads sooner than you would otherwise, so they're always in good repair and the car is always safe.