The garage can easily steal more time than the races. Swap a few parts, take the car out, notice what feels off, then head back and fiddle with it again. Drifting and drag racing have a particularly different feel, so switching between events keeps the driving from becoming too samey.
Items available through the shop can be purchased for free, which makes trying different setups much less of a chore. A few races, a daily task, maybe some multiplayer with friends — the whole thing works nicely as a laid-back car tinkering session rather than a rigid racing grind.