When all soundcards have clocks in them, what is the purpose of these very expensive, separate clocks?
The clocks in sound cards can't always be synchronised to some types of external reference, such as video sync, which is often necessary in audio-visual applications for example.
Also, it is sometimes more practical to use a master clock to drive everything from a central point, rather than chain the clock from a soundcard around a complex system.
For most small digital systems the clock in the soundcard is entirely adequate and can be distributed to the few units that need it without much problem.
And from a techniacl point of view, it is always better to run the main A-D from its internal clock. With only one or two exceptions, running an A-D from an external clock degrades its technical performance, sometimes quite significantly.
Technical Editor, Sound On Sound