Yeah, you usually do not have access to lower level functions most of the time with userland exploits; Kernel = think jailbreak for iOS, or gaining root access for Android/Linux (although that's almost an entirely different subject/exploits with completely different results).
Userland can allow homebrew (bypassing simple security checks) but kernel hacks can open up much more lower level functions for complete hardware control (Higher overclocking anyone? We'll see...) and backup loaders. Kernel = God/Superuser. This, mind you, is a vastly simplified example and by no means is me trying to sound versed in this topic, It's just what I've notice through the years of console modding/hacking.
For example: You use a "web browser" for exploiting some security flaw (i.e. a broken Webkit); since the web browser was in userland (has access to the filesystem, etc.), any exploit spawned from the web browser will inherit the web browser's permissions (i.e. userland). That's why it's usually the first step in hacking and gaining kernel access.
Again, oversimplification. Please feel free to correct me, it's late/early and I should be sleeping.