IT:AD:Azure:Security:Role:Why Move To Builtin Roles
Summary
The Administration Roles were introduced when Azure first was introduced (ie with the IT:AD:Azure:Portal:Classic Portal).
But its simplicity caused issues.
This meant that a subscription typically became a security container.
For example you could provision a subscription for an application or you could provide a subscription for an environment (such as Dev / Test / Prod). Or you could provision a subscription for an application, in an environment. Either way, one ended up with many co-administrators with full control.
Obviously, not a good scenario.
The above issue is why RBAC and Resource Groups were introduced (along with the new Service Portal).
I suspect that now when you are defined as one of the classic *Administrator roles, you are assigned Roles from the built in roles (eg: Owner, Contributor, etc) 1) but maybe not. I have not seen a map of this.