A throttle body service is needed because as an engine operates, it deposits materials from the PCV system into the throttle body which, after a while, will have an effect on how well the vehicle starts and how well it idles in certain conditions, ie.e cold and warm conditions, how much load is applied to the engine. It can also affect how smoothly the vehicle accelerates from a stop.
When a throttle body is service is performed, a technician cleans out the bore of the throttle body and the throttle blade itself. Then, they check and adjust the minimum air rate and as well as the TPS sensor. Then the technician performs an idle relearn procedure. When this is all done properly, the vehicle should idle as if the vehicle was brand new. For vehicles with a computer controlled "drive by wire" throttle body, it can be carefully wiped clean. Then, an electronic throttle body adaption relearn procedure can be performed.
A bonus from a throttle body service is that it keeps the computer controlled idle valve clean which can get "carboned up" to the point where it will have to be replaced, if it is left uncleaned.
Most manufacturers have a factory recommended vehicle maintenance schedule for how frequently a throttle body service is required. This factory recommendation must be adhered by to maintain the validity of your new car warranty and extended auto warranty, if applied. If mechanical breakdown occurs due to the lack of your car's maintenance of the engine oil and filter, the warranty would not cover the car repair.