There are various router bit types in the straight cutting router bit family.  For example, the panel pilot router bit drills its own starting hole while the pilot portion of the router bit acts as a guide for template and pattern routing.  The flush trim router bit with a bottom bearing is used primarily for trimming laminates and other counter top materials while the flush trim router bit with the bearing on top is used for following a template placed on top of the wood piece.  Other straight router bits are used for bottom cleaning or making dadoes which can be frustrating since the wood is always thinner than advertised.