there is some truth to the idea of circulating the qi through the form, but it is deeper than my knowledge. it is discussed by fu zhong wen on his book on the yang 108 as taught to him by yang cheng fu.
as for the segmentation of the form, the simple answer is that the form is designed to be a 'lengthy' exercise. However, for various reasons, once may wish or need to practice only part of the form, and the 'crossing hands' serves as traditional markers of good places to begin/end.
of course, that becomes relative since all the major taiji styles have different divisions , so i guess it can be argued that it's an artistic division based on the form's creator's ideas.