Why?

It handles virtual displays better in case you want your pc screen to be off while streaming. There might be other reasons.

Oh nice, virtual displays is a feature I've been wanting, thanks!

Agreed with OP. It's very handy. I made the switch after trying to tinker with running third party utilities to do this and running into issues. I found Apollo and it all just worked. Now I can stream in 4K HDR to my living room TV (which is not even what my physical PC display is). It's compatible with all the regular clients too which is nice.