There are a couple of ways to address this:
The Preferred Architecture for Exchange 2013 is the Multi-Role setup. This means that both Client Access and Mailbox are designed to be installed together. This is supported.
The only exception here is the Edge Transport Role;this is still on a separate server. -- https://blogs.technet.microsoft.com/exchange/2014/04/21/the-preferred-architecture/
An aside, in Exchange 2016 it's not just preferred but actually enforced.
The other side of the question you pose this is a good one too. "If they're meant to be together why the option to install them separately?"
There's probably some additional situations but here's a couple of them:
ONE, If you're already running NLB and the DAG. If this is true, then the CAS will have to be installed separately from the DAG members.
TWO, If you want to do it. If you do, you'll just have to remember that you'll have more servers to manage. For example. If you're to have two DAG with a Load Balancer (2 Servers) it also means that you'll have two CAS servers too.
THREE, If that's the requirement. Put differently, "I was told it has to be separated."
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