It is about performance.
The idea is that you don't rerun the whole lifecycle at every component that is loaded.
This means that the view instance is not available in a functionnal component, as described in the migration docs from Vue 2 to Vue 3.
In more plain English, this means that the component does not support reactivity and cannot make a reference to itself through the this keyword.
I like this free article from Vue.js Developers publication on Medium.
It is a pretty complete article.
Read the Vue 3 docs for more.
See my follow-along project for this course.