Why does the Linux kernel have its own bootloader

I am trying to work my way through Linux kernel code, and in this file there seems to be a full blown bootloader that bios is expected to run. I was under the impression that generally, grub will be in the mbr and then it will somehow invoke the kernel. My questions are: 1. why is there (to the best of my knowledge) a built in bootloader in the kernel? Is this for historical reasons? 2. Can someone explain step by step what happens from pressing the power button to starting the kernel?