[GMS.VL] just a mercedes i know, doing what mercedes do.

There's a lot of weird things on this SS of yours though, can you explain it? Not accusing you, just looking for clarification

HP washing right after Shinku mentioned the possibility and method to me about 2-3 days prior to that picture.

As seen here: http://v.youku.com/v_show/id_XMzQ5NzM3Mzky.html

HP washing isn't even needed to do one of the hardest bosses in the game. Therefore it is useless.

If you can't see it, the HP is 22.1k with an HP boost potion.

There's several reason to HP wash. Admittedly it doesn't grant as huge of an advantage as it once did pre-BB, but there are still some reasons to.

For example, at empress, I can now use an exception hero + dex boost potion as opposed to using a health boost.
I can also use spikes royale on PB + his minions when he's in reflect.
I also lived in an empress run that I failed earlier today with Villain, I was able to get empress to summon his 5 instructors before I ended up DCing from FJ.

Basically:
Is it useful? Yes. Is it required? No.