Dual polarity vs dual rail power supply
A typical use of the power supply schematic in the O.P. is for powering one (1) analog circuit, which needs both positive and negative power supply rails.  In principle, you can power two (2) completely independent circuits from it (+9 to 0 and 0 to -9).  This would be a peculiar scheme, though, because the ground of one circuit is at 9V (plus or minus) w.r.t. ground of the other one.  Still, if the circuits are in fact independent, such scheme would work.  In practice, this scheme is not used in general purpose desktop power supplies.

A classic desktop power supply (like the one in the YouTube video linked in the O.P.) is of a "dual positive type".  Each channel has its own independent secondary winding (or even a separate transformer) and its own rectifier.  The channels can float with respect to each-other, and that allows to connect them in series.