RE the word Clock,
I have often wondered why that is important. It is important when you're trying to keep many digital process tightly together so as not to have added sample errors build up. Consider this, A lot of structure has been developed on a mixing console and there are a lot of external third party plug ins and if you happened to have any external hardware, it will be advantageous to have an external word clock so as not to allow the host to create additional anomalies that they inevitably do.
No, you do not NEED a word clock as the host does provide it,but it is an advantage. That really only applies to those of us who are using Professional level I/O units. The New Symphony has a Big Ben built in. The Lynx Aurora has a word clock and some or many PCI cards also have word clocks. I am not an expert but I do respect the issue when trying to keep things from running amuck. A big project can do just that especially when dealing with native systems.