Digital BW, The Print

Pardon the newbie question but I am a newbie to these products. I 
print primarily color images on an Epson 7800, and almost exclusively 
on Crane Silver Rag which I find to be extraordinary. I have seen the 
occasional post discussing a spray product that is used as final 
treatment. Could someone please explain the purpose of this process. 
Is is simply for protection of the surface, or does it actually 
enhance the image. Which of these products is suitable for the Crane 
Silver Rag paper. Much obliged.