What is kernel-package? Why we need to install kernel-package? (or Why we have to use another kernel source from http://www.kernel.org)

I have googled the first question but no obvious result appeared for beginners like me. So I gather two important question into a post so that other people can find. Please correct me if I misunderstand something.

According to mannages.ubuntu

The kernel-package package use to automate the routine steps required to compile and install a different (custom) kernel.

So Why we need to install kernel-package? ask.ubuntu

There is a project, started and headed by Linus Torvalds, that maintains the source code for a kernel called the Linux kernel. This is what's distributed at kernel.org. Note that this is not a kernel, it's source code — Like a recipe for a cake but not a cake: To make a cake, you have to follow the recipe step by step, mix ingredients, bake; To make a kernel, you have to configure the source code for your computer and compile it.