Why should we do a bootloader first before the kernel?

You can start wherever you want. And you can use a bootloader ready, as many do. The GRUB is the best known.

As the name says it is responsible for finding where the operating system is on the disk and put it in memory, in addition to making some settings on the machine to start running properly. After this task he can transfer the control to the kernel of the OS that will already be in memory.

In general, the loader does a minimal load on the kernel and then it will do the rest.

If you want to go deeper, you have a website dedicated to building operating systems. Read the bootloader page. It's not easy to do, but it's fun.