It used to be pay per SMS (and not cheap either), that’s why WhatsApp got so big in the first place, it was “free” anywhere with free WiFi. But since many years now, regular SMS became free / dirt cheap (especially within EU), so it’s no longer as good as a reason to use WhatsApp now. Now it’s because it has groups, media, history, is searchable, etc.
It used to be pay per SMS (and not cheap either), that’s why WhatsApp got so big in the first place, it was “free” anywhere with free WiFi. But since many years now, regular SMS became free / dirt cheap (especially within EU), so it’s no longer as good as a reason to use WhatsApp now. Now it’s because it has groups, media, history, is searchable, etc.