A smile? Erm...?
The species it effects most are Panaque and Lasiancistrus, and also a couple of other to a lesser extent such as Hypostomus and Cochliodon. What they get out of it is roughage to help keep there tracts in good working order, but is thought they need the lignin found in cell walls, which is very difficult to degrade. More importantly they get a lot of other things like hemicellulose, polysaccharides which they need. In the food sticky at the top, I think COL mentioned the species that it effects and a bit on bogwood.