An easy way to add custom fields to your object types (post, pages, custom post types, users)
This is also a developer feature, aimed towards site builders. And real developers don't need UIs ;)
(But really, though, the main benefit of this fact comes into play when you're working with multiple environments, i.e. development/local, qa/staging, production. This approach makes it easy to replicate UIs and features without having to worry about database synchronization and other crazy things.)
We're being good and "namespacing" our public functions. You should too.