I always assumed that it was done for convenience. I have 3 hot wires
(A, B & C(wild)), I can run all my 120 stuff off A to neutral or B to
neutral. I can run all of my 240 single phase stuff off A-B and I can
run all of my 240 3-phase stuff off A-B-C.



Anything that uses 220 only, ( 220 with two conductors and a ground) can
be run off of any two legs. Only if the thing uses 120 and 240 volts
and has a neutral wire going to it do you need to use the centertapped
phases.


If I had Wye or corner grounded delta service then I would have to
give up one of those conditions or purchase another transformer.



Nope

By the way: center tapped delta service is very common here in the
Chicago area. Wye service is strictly used in office buildings and
warehouses.



It is good for the electrical company when there is a mixed use of
single and light three phase users. They save a transformer when three
phase is required.


As a side note: I have a suspicion that Commonwealth Edison balances
the overall service in an industrial park by locally grounding the
center of alternate coils. IOW: that wire that comes into my building
and that I call the wild leg, is not the same as my neighbor's wild
leg.

All utility companies alternate on the hookup of the primary to the
three phase high voltage lines, the ones on the extreme top of the pole
with the big insulators.

you neighbor may be on a different pole transformer. IF he is then his
phasing is different than yours.


John