Replacing parts and the service schedule

I am a little confused by the instructions in the maintenance manual for our 3300 gen 4 engine.  It seems wasteful (and expensive) to replace the filters and spark plugs at every annual inspection if you have only very recently done so with a 50 hour or 100 hour.  What is the logic?  Poor weather can reduce the hours flown but the annual keeps ticking along by calendar dates!