Homeowners should test their well water on a regular basis. Well (and especially spring water) can change over time, even suddenly. It can be effected by environmental factors such as weather, age/condition of well/spring components, and sometimes it just changes for no discernable reason.
Total Coliform and E. coli can get into wells/springs and build up slowly over time. For people whose immune systems work properly, their body will build up a resistance to the bacteria.
But if someone comes in from city water, or if someone has immune system issues, it can make them sick as their immune system does not know how to fight it. (See section on Total Coliform for more information).
Many buyers want to test it as part of their “Due Diligence” so they can be aware of any issues or because they need to for the mortgage. Often homeowners will need to test when they refinance.