Bruce in Bangkok writes:

All distribution transformers, sometimes called "pole pigs", that I
have seen had some sort of voltage adjusting system, usually referred
to as taps. Usually they are an actual bolted "tap" and you open the
transformer and set the output voltage by making the proper tap
connection when the transformer is installed and frankly it is usually
ignored thereafter.

What I was talking about appear to be used to adjust for supplied voltage
(they're often used right after a stepdown transformer bank) or long
runs, which may produce somewhat variable voltages that need adjustment
at times.

The other "cans" you often see on poles are capacitors used to adjust
the power factor on some secondaries.

Around here, capacitors for power factor compensation are rectangular
boxes with two bushings on top, on poles, in banks of 3, 6 or sometimes 9.

Like the ones Phil mentioned, the cans I talked about hum.