“why would a site administrator want someone with lesser permissions to be able to give someone else greater permissions than they the giver has…?” is a very good question and that’s exactly why this new feature is required. The default functionality of WordPress allows any user with promote_users capability to promote any user to any role including Administrator. For example if Subscribers get promote_users capability, they can promote themselves to Administrators. This new feature allows you to limit that capability so that they cannot promote anyone to a higher level.
I really don’t think an Administrator will create a public website where anyone can register and promote themselves to Administrator. I would never create an account in such a website, because I don’t want others to steal my information. For example anyone can create an account in wpfront.com by purchasing the product but they cannot promote themselves to Administrator. But if I want a user to have the permission to manage a few other users I can easily do that with this new feature.