Bike shops can save your bacon if there is a tool they have that you don't.
Something like a headset press for example, granted you could make your own out of a bolt and some washers, but its not quite as elegant. Even a bike stand is a godsend, that allows you to focus on repair instead of balance.
Most of all, when you need a bolt or fastener, and it is the final part in your otherwise finished build.
Yes the prices are usually higher, a bird in hand is better than two in the bush.