Why is it specific to forms /checkout

I simply want to add custom fields to members. But the documentation makes it sound like this is very specific to add it to the forms/checkouts and users (why not MEMBERS?!).

I think it is a problem with the confusing documentation more than anything. I don’t even really understand why a separate plugin is even required when the solution is purely custom PHP.

I methodically followed all of the instructions regardless and added the custom PHP but it displays absolutely nothing on the users or members UI related to my new field.