This arrangement provides two major benefits: First, the elimination of all the errors associated with the improper figuring of a parabolic reflector.  Few, if any, manufacturers can afford the extra time, care, and effort required to properly figure a parabolic mirror; making coma error and spherical aberation very common among mass produced telescopes. The spherical reflector does not have to be "figured" so it never produces these aberrations.
Second, this design has all the advantages of a Newtonian design over a Cassegrain design, namely it produces a much smaller obstruction to incomming light rays. This is due to the Flat's location closer to the main mirror's focal point where the light cone is much smaller, and due to the fact that a secondary mirror oriented at a 45° angle to the incomming light stream will produce a horizontal obstruction that is 29.3% smaller than the same mirror would if it were oriented at 90° to the light stream.