These posts have pictures of some good examples of pocketing.
http://www.chiefdelphi.com/forums/sh...stcount1387268
http://www.chiefdelphi.com/forums/sh...stcount1396742
http://www.chiefdelphi.com/forums/sh....php?p=1308653
It's often used because a flat surface is needed (or is convenient for fabrication) or the thickness is needed for bearings or geometry, and then you cut away material to get a truss structure between the important (load bearing) points in the part. Reduces weight, can ease maintenance, and also looks kinda cool.