To be fair though, while Whatsapp is the default messenger for pretty much everyone, both signal and telegram also have their users, especially with younger people. Eg the volunteers for helping the Ukrainian refugees in my city organize via telegram, and the above-mentioned bikeshop ended up messaging me via Signal.
At the time Whatsapp came out, I was in NYC, and I do not recall anyone not using WhatsApp. I remember how amazing it was to effortlessly chat with my UK family since we were no longer subject to super expensive international SMS/MMS charges.
Best use case is at a wedding or event or on vacation when you want to share pictures/video with others. Simplest mechanism is to send it to them via iMessage.