I am wondering why people still need root rights in these days.. I had Magisk installed on Android 6 last time... You just messed up whole system by doing something what you have no jdea about..
I see people making this "argument" on other sites, but I really did not expect it to be told by the admin of a custom ROM site. I've been using android almost since it was released to public, heck I've been a member here since 2010. The argument you are making is just like saying why would you install a custom ROM you will just mess the whole system by unlocking it. Why would you even want the ability to change anything the manufacturer knows best.
Since you asked, here are some reasons why I rooted every android version I used since Cupcake (2009): customization (people have different tastes and different use cases), learning and reverse engineering closed sourced platforms (just an example: developing Xiaomi integrations for otherplatforms like HA), privacy ( for example xprivacy and xprivacylua), having additional features and functionality (viper4android lets me tweak my headphone's sound to exactly what I want), circumventing geo-blocking, ad blocking without having multiple services running in the background the whole time, etc. Every single one of those reasons are still valid today as much as they were back in the old days.
If the user does not know what they are doing, anything can mess something up. "A disclaimer that you can mess things up if you don't know what you are doing" and "why would you do it, you will just mess it up" are two different things and one of them is objectively wrong.