What is your use case for degrading image quality that much? At quality level 80 the artifacts are already significant.
If you must use JPEG, I suspect you might get better visual quality by halving the resolution and upsampling on the client.
By doing so, you are effectively setting the lower and right halves of the DCT to zero (losing all high resolution info), but get to have 32x32 pixel macroblocks which lets you better make use of low frequency spacial patterns.
There are infinite use-cases for everything beside one's own tiny personal experience and imagination. It's not remarkable that someone tested for the best version of something you personally don't have a use for.
Pretend they hadn't answered the question. The answer is it doesn't matter.
They stated a goal of x, and compared present-x against testing-x and found present-x was the better-x.
"Why do they want x when I only care about y?" is irrelevant.
I mean you may be idly curious and that's not illegal, but you also stated a reason for the question which makes the question not idle but a challenge (the "when I only care about y" part).
What I mean by "doesn't matter" is, whatever their use-case is, it's automatically always valid, and so it doesn't change anything, and so it doesn't matter.
Their answer happened to be something you probably agree is a valid use-case, but that's just happenstance. They don't have to have a use-case you happen to approve of or even understand.