So that’s the what, now for the why:
When the 1972TS were procured, the practice of uncoupling during off peak hours still took place on the Underground, with shorter units running between the peaks allowing for increased maintenance time and decreased wear. This was a practice that had been around since the earliest days of electric traction, and (arguably!) didn’t finally die out until the Chesham shuttle ended in 2010.
Which, of course, hints at another reason for shorter units that could be coupled together. Although not specifically relevant to the 72TS, several lines also operated self-contained shuttle services requiring shorter than full length trains - Aldwych, Ongar, Chesham and Woodford-Hainault all spring to mind. Indeed, the latter was assisted by the Victoria line’s 1967 stock being designed as four-car trains, despite being intended to always run as full length 8-car units. The design heritage of the 1972TS (essentially a manually-operated version of the 1967TS) gives some indication as to why this is also in ‘unit’ format.
Finally, it is worth noting (although implied above it’s good to clarify things) that originally the middle cabs were fully operable as with the outer cabs. Refurbishments and new technology over the years has resulted in these not being upgraded to the same standard, and now cannot be used for passenger-carrying trains (on the Bakerloo some are maintained sufficiently to allow test running or stock transfers on the mainline, although I am not aware of any examples of this actually happening within the last decade).