You actually can boot without one—the system I’m typing this on right now has none. However, you have to keep the system simple enough that the root partition can be mounted without requiring anything not built into the kernel. That means your root partition can’t be encrypted or RAIDed, and you can’t build the support for the filesystem it uses as a module, which can mean a custom kernel. Most people don’t want to bother.

Per the “ancient” “Chinese” curse, “May you live in interesting times,” I suspect.

(The phrase is actually the brainchild of a British diplomat who was posted to China for a while sometime around 1900, if I recall correctly.)