AS Border Router and Core Router Separation Required?

Hi, I am learning for small ISP network operation and I have a question regarding a basic AS BGP operation.

A lot of example AS network topologies I see online have Boarder Routers (ASBR) and Core Routers (CR). ASBRs are connected to IP transit providers or AS peers (or IX). On the other hand, Core Routers are connected to ASBRs and servers/clients.

My question is, why do we need to separate ASBRs and Core Routers? For example, if we have a basic network with 2 IP transit uplinks, we could have 2 routers with IP transit connected to each router to have fault torelance. So why should we have 4 routers (2 ASBRs and 2 CRs)?