If we had to pick just one part of your plumbing system that was essential to the entire operation - it would be your drain system.
Each fixture, component, and appliance within the system is important but, none of them can work if there is something going on with the drain system.
In fact, it is nearly impossible for a toilet to flush if there is a clog in the drain. And, you can’t really continue preparing dinner if your kitchen drain is damaged and leaking water all over the kitchen floor.
The drains in your plumbing system are the backbone of the entire operation - and they have to be well cared for or you could risk contaminating your clean water source.