As reflecting in many comments already, they enable those of us who managed a series of databases that cross many applications to manage security extremely easily. No longer does each source file(proc, view) have to have grants in it, or any creative work to set security. Schemas(namespaces seems to be easier for them to understand) to simplify those pieces.

ordersystem.pr_Get_Order is just easier to manage than dbo.pr_os_Get_Order (or whatever) from the standpoint that schemas make permission management so simple.

One app per database? Overhead - 10 apps sharing 7 databases, big gains in efficiency (and self documentation)