Oh, I don't know let me try and help you out.
Do you own a car? Do you have a spare tire? Do you carry jumper cables?
Do you scuba dive? Do you have a secondary regulator? Spare air? Whistle? Flag?
Do you rent a home? Own a house? If do, do you keep spare lightbulbs? Toilet paper?
Do you sky dive? Have an emergency chute?
Heard of NASA? Yeah, they are big on redundancy.
Aircraft? Yeah - takes two engines to cross the Pacific or Atlantic with passengers.
Military? Yeah, we love spares too. Things like WRISK kits fundamental and I won't even go into air frames or personal gear.
So to your point about not understanding how anyone can have a system that you "have" to keep a spare is rather silly. A properly designed system will have some form of spare parts or redundancy built in. It can be in they system but not in use as a hot spare or it can be in a box for hot swap. If you don't I'd wager your chances of a failure are 75% if not higher and a total loss.
C19 alone should have made people pay attention due to supply chain difficulties, transportation priority changes, and various levels of lock down / shelter in place making overnight replacements scarce.
Note: This does not mean double of everything. What it does mean is that critical components be it a heater, pump, light, water, salt, or a container large enough to hold live stock should a tank fail be considered in case the hobbyist not being able to replace it in due time.
Don't take this the wrong way but this is pretty much common sense to me.