When a stranger, acquaintance, or even a friend borrows your phone to make a call, you want to make sure that they don’t go opening other apps (accidentally or on purpose). Android lets you do exactly that with “App Pinning”.
It’s also perfect if you want a small child to maybe play a puzzle game etc on your phone.
On my Samsung Galaxy phone it was disabled by default, but the linked article’s advice worked to activate it and to use it.
See https://www.howtogeek.com/always-do-this-before-handing-your-android-phone-to-someone/