It doesn't matter for the users at all, of course. But I'd think that phone booths have a few benefits that makes this a good idea.

For one thing, there's a phone connection right there. I imagine that they would plug into this for the hotspot.

Plus, they don't have to figure out where to put the hotspots: just use the booths, period. It would probably make the deployment simpler and faster.

Also, the overall coverage will most likely not be perfect, so the booths are a visible structure to get close to if your signal is weak (as opposed to try to spot a standalone router for example).

You don't get access to the phone line on anything without paying extra money to put a WiFi router there, especially in high-priced locations in city centers. Reusing phone booths eliminates that cost.

The idea sounds cool, but I doubt it's more wishful thinking: it was pretty clear at least six years ago that mobile is the feature and no major landline operator (actually there was only one that matters before it was “split” into two) spent money building or maintaining those phone booths. The # of photo booths is really negligible in many cities.