Can someone explain Video format in Project Settings

I've read the manual and watched a YouTube video about Video format in Project Settings and I'm still confused. What does it relate to? If I change it to anything I see nothing change. Whether it's 8K or PAL 16:9 I don't see a difference.

My screen size is 3840x2160
Timeline resolution is 4096x2160
Timeline frame rate is 25
Playback frame rate is 25

What do I set Video format to under Video Monitoring?
UHD 2160p 25?
4K 2160p 25?
HD 1080p 25?

What is the point of this Video format option?

It's about external video monitoring, via an UltraStudio or DeckLink card. Typically you'll want to match your timeline - so 4K 2160p 25 in your case, but it matters only if you have an external monitor.