let me try,
let's say u r doing wireless communication. u want to send out data thru dac and the other side receive the data thru adc.
here's the question, how do u send out the following data: 00 00 00 00 00 00 00 00 ?
if u don't whiten the data, u'll send out nothing thru the dac, and the other side don't know what u are doing.
by whitening and de-whitening, you can make sure u send out something and the other side can  receive what u've sent

Thanks for your guys' replies, could you recommend some good references regarding this issue to me? Your suggestions and recommendations will be appreciated.

-Terry