Because suppliers do things differently.
1. They give you a portal, user and password.
2. They take your MAC address and give you their portal.
The benefit of the first is you can use your line on many different apps and devices(not at the same time) ~ you could for instance put it on your phone and your new A9, and as long as you only use one at a time, you can use both.
The downside of the second is that you are tied to that box/mac.
That's how I see it anyway.