I also have an HP-71B, It has HP-IL and can control external devices. HP made an HP-IB (IEEE 488) interface for it, which I don't have.
HPIB on a calculator ?!
I wonder what use case HP was thinking of for professionals to want that ?!
Maybe because the bus is so old, late '60s / early '70's, no laptop back then, so using the calculator with an interface was like a portable solution to go work on HPIB gear on the shop floor ? Useful for technicians to set up and maintain devices " in the field " / on site ? You could make a iittle test program on the calculator to do some basic trouleshooting ?
But I guess htere must have been better solutions even back then, some portable and dedicated piece of test gear custom designed to work on HPIB stuff...
But well, it was probably very expensive so the calculator with an interface was still a good solution for simple needs and lower budget. Lighter to carry around as well I guess.