I am trying to understand the process. Linus et al maintain the "official" kernel, deciding what patches get in, etc. Then a distribution takes the official kernel and adds its own unofficial patches... because... why?

There are a couple of common reasons to modify the kernel for somebody making a distribution. The biggest reasons are compatability, security and adding features. Adding features to a distribution that have not been included in the base kernel is one of the common reasons for the more specific distributions. Distribution vendors are also usually faster to include security patches then the kernel, particularly if the problem is theoretical or the solution is ugly.

But compatability is the probably the single largest reason. Distributions often include patches to make a kernel work with binary drivers or to fix problems with certain software or make it work with their distribution specific systems.

Shouldn't the official kernel be sufficient? And if each distribution modifies, sometimes markedly so, the official kernel, then what is the official kernel used for? Is it simply a standard, or is it actually used for something? I know these are very uninformed questions, thanks for looking and thanks for any light you can shed on this for me.

The only reason most people might want to use the official kernel is that they want to customize the kernel or they want to be involved in testing the working development kernel.

Jay