In most cases, a check engine light does not mean that you need a tune up. This is a common misconception. At times, there can be misfires which are caused by bad spark plugs. When that happens, the vehicle will run poorly as well as causing a check engine light. However, 90% of check engine lights are for other reasons. So to just assume a tune up is wrong about 90% of the time.
Tune ups should be done for maintenance before a problem surfaces. The definition of tune up can be (and usually is) greatly debated.