By using a flat bottom endmill and maintaining perpendicularity to the complex surface you can step-over the full diameter of the cutter thereby dramatically reducing the required number of passes across a surface. The same principle applies to sidemilling of angled surfaces.
Using a flat bottom endmill to maintain perpendicularity to the complex surface can eliminate ribbing caused by ball-nose endmills.
Machine complex parts that are not otherwise possible, including holes required to be normal to a complex surface.
Eliminate costly tooling and fixtures required to hold the work-piece in place.
Eliminate manual millwork and handwork required to cleanup kellered surfaces.
Eliminate multiple setups required to re-position the work-piece at complex angles.
Fewer competitors means a larger profit margin.