I'm not an electrical expert- by any measure. This section (Single Phase Power Systems) is tremendously helpful and well-presented. I am confused about the "in-phase" and "180 out of phase" in reference to the source voltage from the center tapped secondary coil. Earlier diagram shows both 120V sources at 0 phase angle, + and - notations in "series" with a resulting in 240V potential at leads. Later diagram shows upper 120V source at 0 phase angle, lower 120V source at 180 (however text specifies both are at 0 phase angle), + and - notations in parallel (- to - anyway) resulting in 240V potential at load- seems inconsistent.
Earlier text also initially indicates the 2 120V supplies are "in phase with each other!" and later text states "we must express their relative phase shifts as being 180 degrees apart".
My understanding is that the two 120V source voltages are indeed phase shifted 180 (relative to the neutral??) to produce 240V potential across the L1 and L2 leads.

Very possibly my own lack of understanding but that's my 2 cents. I would be great if a representative sine wave(s) form graphic was presented to show the relationship of the two source voltages and how they "boost" one another to generate 240V.

knn

 

Give reference to what your read, as we can't guess.

 

Vol II - AC

http://www.allaboutcircuits.com/vol_2/chpt_10/1.html

 

In both picture the two AC sources generate 240V across the load.
The difference is in the description of the procedure. The result is the same.

You are correct to say that in this example, the two methods are superfluous and maybe confusing, but as the book says, this way of perceiving it is useful in multi-source systems.