Firewire 400 has enough bandwith to handle the I/O, so no need for a faster interface
In the case of the UCX the DSP processing is applied for the built-in effects in the sound-card (mainly EQ and comp) which can be useful in many different situations (recording, mixing, but also as a standalone audio processor). It's invaluable if you have to handle several monitor mixes for example. But these are not plugins that you can use in your session, they are effects that are applied directly to the inputs and outputs of the interface, much like digital hardware peripherals. And they are not VST, even though Steinberg gives you a VST equivalent.
Theses are sound interface and not DSP accelerators (such as the UAD-2 e.g.)