Different people have differing opinions. Some are based on experience, some on intuition. Some people totally miss the context of what is said. It's that way on any internet forum.
I don't see why you say a "MUST do"? Can you reference where you see someone saying must?
In my opinion the viability float test is a last ditch desperation measure for people where the eggs are long overdue but they just can't bring themselves to toss them. Usually first timers. If the egg wiggles at that stage of incubation the chick is still alive inside. If it doesn't wiggle they don't feel as guilty about tossing it. If the egg is not cracked or has not external pipped you will not drown the chick inside. It only takes a few seconds, the egg is not in there for hours. This late in incubation you are not worried about washing the bloom off, bacteria won't have time to get inside and grow.
I don't consider it necessary at all. The only time I tried it was to help my niece who was visiting to let go the hope of the last egg hatching. I think it did help her, she is a very gentle soul. But when I did that test she was OK with letting it go.