Kinda just like Atmos on a soundbar really...

Its up there with slapping "THX" certification on a Gaming laptop.... :)

its simply branding and product differentiation....

I have it turned on, but I rarely watch content for prolonged periods on my phone and I can't say I've noticed much difference. 

Guessing it is for when you connect via bluetooth to a home theater for music or videos. Although a wired plug will probably provide better audio quality that those types of volumes. I thought Atmos was though sound that was above you, so unless it is connected to a home theater where the  speakers direct sound upwards to bounce off the ceiling, I can't see how it works. 

On the Huawei P20 it has made dialogue a lot easier to distinguish from the background music.

ShinyChrome:

 

Kinda just like Atmos on a soundbar really...

 

Except that some soundbars do have additional upward-pointing speakers to give the height dimension. I haven't seen a mobile phone with anything like that but maybe there are mobile phones with additional speakers.