Useful for Utility Power connection to convert 3ø (120 degree phase angle, 3 wire) to 2ø (90 degree phase angle, 3 or 4 wire) and allow full use of all three phases.
Scott T-Transformers can be built to transform voltage as well as polyphase (such as 480v delta to 120/240v), but that does not appear to be as useful in most applications we consider here.
For further background.
Possible Sources: