Sometimes a building with an existing 3-wire single-phase service will need one 3-phase motor, for whatever reason. It's too much expense to rewire the whole building. Add one transformer and one wire, and you can supply that special load. This is common in auto shops. You need 3 phase to run the lift pump, and maybe an air compressor. Everything else is single phase.
It might also be common in rural areas served by single-phase. If you need 3 phase, and the primary run is long, you only need run one extra wire.
The whole scenario of being able to do with two transformers in an emergency where one burns up might just be a rarity. Probably half of the deltas I see around here are "open deltas" to begin with.
Here in Austin, there are a few large houses built in the early '50s with center-ground deltas. It seems that the earliest central air conditioning systems required 3 phase. Usually, there will be 2 weatherheads, one for the 3 phase load, and one for the single phase load. I changed out one where the 3 phase had long since been disconnected. It had a huge metercan for 2 A-base meters, but only the single-phase meter was connected. Of course, the new service I built was single phase only.
There are many areas where utilities do not supply delta connections, and I'm not sure they exist at all outside the US.
