Triad isolation transformers?....the small audio ones

[redrabbit]

I picked up a few of these at some auction website .
Triad SP-68


But I don't really understand where they are commonly used, application wise.
I have cleverly avoided transformers so far.

Can you steer me in the right direction ?

=RR=

 

[speakerguy79]

Telephones maybe? 300Hz lower limit is about what the phone systems do.