I think it's a user-experience thing. If a user-level application (eg, browser) discovers an available update, it has to launch a privileged process to update itself - which means an authorisation prompt. And just popping up and asking people for their password without an obvious reason why is bad form, so you usually end up offering to update. It makes the whole process more invasive, less seamless, and more chance people will click no/cancel.

A background agent can do this invisibly; no prompts, no interaction. It's much more seamless, but it does require a bit of arrogance on the developers part - it seems quite silly that every application would have such an agent, so they're self-selected by self-importance.