Do USB & Firewire ports on audio interface always function as both input and output?

I guess everyone must start somewhere, so although it's assumed by almost everyone that I would know the answer to my questions, maybe with good reason, what I don't know is whether the single USB connection and firewire ports on an audio interface ALWAYS function as both an input AND output. In other words, whenever I read product info on audio interfaces, no matter where I go, it's generally assumed that most people are buying their audio interface for RECORDING. And so when most people talk about hooking up their Apple computer, iMac or MacBook Pro, it's generally assumed they look to the USB port as an INPUT. That's all very well and good. But in my case, I want to use the USB port as an output (not the mini plug), and go into an audio interface that gives me a balanced signal output that I can plug into my powered studio monitor (which only has an XLR balanced input). All the examples I see with audio interfaces address recording, and imply the use of the USB on the audio interface as input.

So my question is: Does the one USB port that I see on any number of audio interfaces always function as BOTH input and output? It must, but if so, why doesn't any site mention that this fact, and whey don't they show in the manual any diagrams of the audio interface hooking up to studio monitors? I realize it may be obvious to some, but as a user intending to use not an all in one stage piano but rather a controller keyboard that is attached to the iMac in order to use virtual instrument software, I need to go from the controller to the iMac and then out of the iMac into the balanced inputs of the powered monitors. Getting to the balanced inputs of the speakers requires more than a simple adapter to give the speaker a balanced input. But nobody talks about audio interfaces generally unless they talking about it as a way into the computer to record. What about my situation? Why don't they include that example? And why should they assume that novice will automatically KNOW that the usb port on an audio interface will function as an output also if they never EVER mention that example or setup? I assume it has to function in both direction! But really, am I crazy to wonder when nobody ever discusses or shows this setup? It was suggested that I buy something similar to a Steinberg UR22MkII which has a USB. Even the Steinberg site only talks about recordings, and therefore using the USB as an input to use with recording software. There is no mention of using it to come OUT of the mac into balanced speaker inputs, even in the manual. Actually, that's issue is always true of every audio interface manual I have looked, even by other manufacturers! Why do they all assume a novice such as myself (whose money is just as good as the experienced user's money) would KNOW that? It's frustrating!

I realize this is not strictly a Logic question, but I am assuming, I think correctly, that a Logic user's community might be more appropriate for me to address my question to than other communities. If I am wrong, please help re-address it elsewhere. Thank you.