broadly speaking, the recommendation to put bleach in water is to protect against water-borne illnesses - parasites and bacteria, mostly. hepatitis and polio are also known to spread through water, although the water is just a medium for the real vector, which is......shit. well, we still dump it in the river in a lot of places...

if you think there's polio in your water, i'd say you should find a different source of water. if you can't? well, that's what the rec is for.

is there any reason to think that something like this would be useful for covid-19?

well, we've found it in feces, indicating it is potentially in the water supply in some places. but, you'd think that the things we're doing to get rid of other viruses would be largely useful for this one, too. and, what do we do to our water at the purification plant?

the answer is that we basically bleach it.