Interchangable mesh hair and particle hair, or something else?

I’ve been wanting to do some hair simulation for a while now, so I have recently been experimenting and messing around with it in Blender. To get some ideas, I was looking around the web for 3D hair and how Blender and Pixar does it, and I found this:

The first photo shows a character from the Blender Gooseberry project with 3D simple mesh hair. The second shows it all rendered with particle hair. My question is, is the mesh hair just a proof-of-concept mesh hair that is used during animation to show how it’s supposed to look, or is it actually used to help the particle hair to stay in-shape or something of the sort?

I saw this used also is Pixar films, see This B-Roll for an example. It also uses 3D mesh hair, but the final film uses particle hair.