I heavily utilize NPCs in my character as well as utilize a hybrid style of them on a number of my open world threads (which is my preference generally).

I actually started utilizing a system of allowing people to create really weak shell bios for what they needed to get done for random NPC level characters to allow them to participate before they completely flesh out their bio (this works great for those on the fence with a character concept). It has worked quite well for my open world threads in creating a wider range of characters, even if I am the only that mostly utilizes that feature. 

A lot of my NPCs have actually expansive bios and I'm often post them on threads. As has been mentioned, they just make the world feel much larger and that is often a struggle for a lot of more open world threads.