Great question! I'm glad you asked. :) The best way to answer that is to describe what is done and what the outcome of each service may provide.
1. Your refrigerant levels are tested and based on the type of refrigerant you have, refrigerant may be added or removed from the system to ensure it's operating at optimal efficiency. This not only will extend the lifespan of your system but it may also significantly lower your electrical bills. A system operating on too little refrigerant will run for much longer than it should and often times never turning off leading to motor and/or compressor failure eventually. You might pay off the cost of servicing in just the first couple weeks of your AC usage.
2. Your outdoor coils are cleaned and your fins are straightened within reason. Dirty outdoor coils or fins that are severely bent can lead to poor heat dissipation and therefore an inefficient running system. Inefficient running systems are more expensive to operate.
3. Your plenum is reviewed for leaks (very common!). Why are these leaks bad? Because you're paying to cool your attic and unless there is a furry critter up there you're trying to make more comfortable that's a poor use of resources.
4. Your capacitance is checked and compressor currents/voltages. This gives you a good idea of how well your compressor is operating and if it's nearing the end of it's life. That is useful because most often people who are pro-active in getting quotes for a system get much better values than those who wait until their system altogether fails and in a place of desperation being in the middle of summer.
5. There are other system checks that all align with the same goal of maintaining an efficient system that will last as long as possible. In short, it is well worth the money.
Now is the best time for the annual checkup usually cost run around $50-$100 depending on the system. If you don't know who to call. Please contact me I'll be happy to recommend some great companies to you.