Apart from the unavoidable ravages of use and friction, the greatest danger to your machinery is damage to the material it's made of – rust being the prime culprit. Many machines which would otherwise still be able to provide years of good use have to be scrapped because of corrosion.
It's very hard to prevent the metal surfaces of hardworking machines from being exposed to air and moisture. Protective coatings get pierced or worn away over time. Once the metal is exposed, it is only a matter of time before corrosion sets in. Making matters worse is the fact that almost all machines are used in exposed areas where they cannot always be protected from moisture.