Digital BW, The Print

Friends, I print both color and B&W on an Epson 7800. I have been 
using Crane Silver Rag since I got the printer. My Epson 2400 
favorites were Velvet and Ultrasmooth. I have been reading recent 
threads which describe Art Shield and other finishes as the final 
touch. I am not familiar with this process. Why is this done, is it 
effective for both color and B&W. Are there particular images where it 
would be most effective, ie: more blacks or "bald" highlights. Thanks