An important fact hinted at below is: you got the bill as part of your monthly phone bill.
Probably the most common use, not all that interesting to us, was replacing 411 (remember that? I don't think that was the Information number in France.)
Besides porn, it also had online dating.
In Brazil (São Paulo) we had a similar, Minitel-based, system. No online dating, but I met my first wife in a chatroom. It also had banking and you could transfer money to different banks with it. Initially I got an MSX computer with a modem (at that time the telco rented out the computer instead of a dedicated terminal for a ridiculously low fee), which was also useful for connecting to local BBSs.
Later on I gave the MSX back (should never have done that - it was an unremarkable computer, but a pretty good videogame) and added a V.23 modem to my Apple II+ clone. The system continued to evolve, started accepting 1200 and 2400 bps inbound connections (which made it more accessible, because V.23 modems were not easy to come by). Last time I used it was with Windows 95 and its terminal software.
Legend says the system ran on Multics on a Honeywell-Bull mainframe, making me an actual Multics user.
Also in France, 15 is medical services, 17 is the police and 18 are the firefighters. By calling 112 anywhere in Europe you'll get an English speaking operator that can dispatch any emergency service.