> Okay, so ELI 5 why I would now use HDMI instead of DP?

I use DP too when I can. For example my main desktop is using DP.

But many devices have HDMI, not DP. For example I've got seven Raspberry Pi here: they all have HDMI and none has DP. There's, what, 100 million Raspberry Pi in the world by now? Don't know the number but: devices that do not support DP aren't rare.

And Linux runs on much more than desktop PCs.