These were everywhere in the 90s around London. The amount of places you could go to for 5p was staggering. They came in (from what I can recall) around the time BR was gearing up to privatisation and most stations in SE London were totally unstaffed for most, or all of the day and revenue checks were almost never. Every station seemed to have one, although some were quite well hidden. When did they start to disappear? And are there any left?
Network Southeast installed one at my local station at the time (in sussex) by late 1980s. Before privatisation was on the active agenda really.
The station had part time ticket office hours, but no other ticket machine (busier NSE stations did have an early era version of a ticket machine with buttons for popular local destinations IIRC*). I think they came in with the advent of penalty fare scheme but for stations that would not have been open to allow you to buy a ticket, to show that you were not intending to travel without a ticket (of course).
I would use it regularly, then go to find the guard on the train to avoid a quene at the excess fares window on arrival at the terminus.
I recall the machine was removed around the time the TOC installed a sophisticated TVM that would sell a fairly wide range of tickets.
On one occasion the 'permit' it printed out was blank - machine run out of ink I assume, so I got 2. I thought it might be handy to keep one as a fall back if I was ever in a situation where I'd not got a ticket and didn't want to encounter a prosecution happy inspector! Such a situation never arose, as I'm pretty sure that 25+ years later I still have the blank permit to travel!
* yes, the Ascom8050 by the looks of it - this says from 1989
https://en.wikipedia.org/wiki/Ascom_B8050_Quickfare
so I'm guessing the permit to travel machine would have been installed at the same sort of time the ticket machine went in or before, in case the Ascom TVM did not have your ticket on the button list, or the station did not have the Ascom m/c installed at all.