That cliche has a wide variety of applications, from sports, to mechanical, to art and music.
It is widely accepted: you must exercise whatever assets you possess. Would that also include your plumbing system? Absolutely! A quick tale, (not a tall one): I was in a very sturdy and well maintained colonial style home. It had three bathrooms along with three bedrooms. The only rather odd fact was, it was only used as an office in the basement and a small interior room on the first floor. This large well equipped home was not inhabited by a family. As I was told to check all the bathrooms for operation I saw a very familiar thing, (to plumbers that is) : there were little large winged flies in all the bathrooms. These are called in layman's terms "sewer flies". Where did they come from? Each plumbing fixture has what is called a "trap".
It is a "u" shaped pipe that holds water. Its purpose? To keep out sewer gases and sewer flies!
The fact is, because there was no-one using these bathroom fixtures ( save one first floor toilet and vanity ) these traps dried out. They were not used so the traps lost their seal; cliche rings true again. Other problems arise from lack of use. Seals on toilets can dry and crack; diverter valves may not mix properly; washers can dry and leak etc. What do we learn? Even if we do not need to use all the bathrooms in our home, make it a practice to use each fixture at least every two weeks, keep things flowing! A famous sitcom once had a character named George with the assignment of "exercising the gaskets" on his friend's toilet, funny but entirely practical.
   Remember "if you don't use it, you'll lose it".

     Thanks, J J