do you not go to the movies in the US? Fandango is one of the only ways you can prepurchase tickets - their kiosks are at every theater I've been to over the last ~10 years.
I think many purchase online directly through the theater. I used Fandango until I started using AMC's app directly, since most of the theaters I'd want to go are AMCs.
Canadian here, Fandango's one of those names that you hear and it rings a bell, but chances are you haven't really interacted with them and don't know much about them beyond them being in the ticket sale/resale space broadly.