Each account/Subaccount can have up to 25 Users. If your Inbox is full of simultaneous two-way conversations, it's important to know which User is responding to which message. If you want a separate Inbox for a specific ministry, you can create a Subaccount. Each Subaccount will have its own Inbox and its' own Users managing it.
In the message thread view, you can see which User sent each reply.