Why does "Passcode" or random apps disappear when I adjust the Settings on the "Screen Time" function?

What is the purpose of Screen Time?

How do I utilise it effectively and efficiently without my "Passcode" function vanishing and without my all applications disappearing from "Home" screen?

(I do not have kids by the way, but I want to be conscious and mindful of how I use my time and energy as an adult so Screen Time sounds like it would do just the trick!)