These small transformers were either for street light operation or were metering transformers on one of the utility's primary circuits. If the latter (as I believe) these units stepped down the primary voltage (such as 2,400 v) to 120 volts and also provided the proper current for flowing through the meter (that might have been inside the wooden cabinet on the backside of the pole). Metering transformers stepped down the voltage (to 120v) and proportionally for amperes for measurement taken from primary lines. Otherwise it would be virtually impossible to build a meter with primary voltage going straight through it and insulated enough to properly do the job. Such installations are very common, particularly for large users.