It's not a tremendously accurate simulation by any means, but it's great fun nonetheless, and it might even teach you some of the basics about how cars are put together. The repairs themselves usually follow a standard plot: run diagnostics (test drive, OBD checker, etc) or examine the parts yourself until you find the one or two parts that are bad, remove as many parts as are necessary to remove the bad parts, buy replacements for the bad parts from your garage computer (or repair them if possible), then install the new parts and replace everything else.
At least, that's how it's *supposed* to go. If you want to go a different (*cough* chop shop *cough*) route, you can make use of the otherwise mostly useless used parts store also on your garage's computer.
Say your job is to replace some bad brake pads. To get to those pads, you'll also have to remove the tire and the brake caliper. The parts that you don't need to replace for a job are usually in pristine condition, meaning that if you sell them, you'll get a much higher price for them. All you have to do is go to the used parts store and see if they have a working caliper and tire for that car that's in okay condition (as long as it's better than red condition, it should work). Of course, they might not have a barely working caliper or tire for that car, but they might have an exhaust manifold that will be just perfect. So, remove the original exhaust manifold too. As long as it's in better condition than what the used parts store is selling, you'll profit. Sell off the originals, buy/install the used parts, and pocket the difference. Your clueless customers will drive off happy, and you will have sold off perfectly good parts of their car for a tidy profit.