I’ve done many services recently where I’ve been told at least one of the bikes requested for a tune-up probably “doesn’t need much”. This is the perception for many customers with a newer nice bike with shifting working just OK.
However, it’s not uncommon that the bike “not needing much” ends up taking the most service time. I often find the brake pads full of aluminum rim flakes, slightly mis-aligned brake pads, cable housing that could be much smoother with new lubrication, a frayed cable end, crank bolts looser than the manufacturers recommended torque, dry seatposts approaching seizure in the frame, shifting that works but could be much crisper, and so forth. I’ve had many clients amazed and how the bike feels after a tune-up which usually provides very tangible results; but the tune-up is also important to delay the slow aging of the bike and prevent other problems.
This is why a tune-up of often used bikes is highly recommended once a year.