Why have a heater tune up?

Furnace manufacturers all recommend annual inspections and maintenance by a qualified technician. They also have language in their warranties saying that damage to the units caused by improper maintenance is not covered under warranty. Does this mean that an annual furnace check-up is really required, or the warranty is voided? Probably not, but it's recommended. The best analogy I can think of is going to the dentist every six months for a check-up and cleaning; probably not necessary, but recommended.