Adds the ability to have different themes available to different NETWORKS in a multisite atmoshpere.
I was providing clients with wpmu/wpms sites and I needed a way to make only certain themes available to certain network admins.
This is a common misconception. Technically it's not. Remember, a network is different than a site. A network contains sites. A network can be at Domain1.com while you have another network at Domain2.com, both with different sites underneath of them