My understanding is you can only use the foam once
That’s correct.
more importantly, using the foam means no chance of repairing the tyre
That isn’t.
I’ve used foam twice now, and the tyre was repairable both times. They just cleaned out the foam and repaired the tyre as normal.
It’s easy to use, and simpler than changing a wheel by the side of the road.
The downside is obviously that if you have a proper blowout it won’t work.
The upside of that is that if your car wasn’t supplied with a spare when new, the AA/RAC/Green Flag etc, will arrange a mobile tyre fitter to come and fit a new tyre if needed.
Granted, that’s not as fast as fitting a spare, but its an option.