Have you ever gone out of town for a couple of days, returned, and discovered that the filter you have on your tank(s) failed at some point (e.g., just stopped working)? Many aquarists rely on a single filter to both clean the water and provide surface agitation. If you’re in that group and the filter fails, your fish will suffocate long before they’ll die of poisoned water. So how do you address that? Filter redundancy.
I come across aquarists all the time who’ve lost fish because of a filter failure. This is one of the reasons I use two filters on all my large tanks (< 40g). If I go out of town, I don’t have to worry about filter failures because the probability of losing two filters at the same time is significantly low.
So what about the smaller tanks? I don’t have as much invested in them from a livestock standpoint because those aren’t my show tanks. My smaller tanks are for hospitalization, quarantine, or fry grow out.
I worry more about my fish running out of oxygen than I do the effects of bad water.