Hangouts was great, used xmpp in the beginning and was available on most devices so would work across android, iOS, blackberry, Mac, windows and web. They screwed it up and WhatsApp won.
I remember thinking how genius and novel the approach was, and all the people in my family, even the old, non English speaking, non computer literate people immediately knew how to use it. All you had to do was point people to the app, and they needed no more hand holding.
Of course, it was also a great cross platform user experience, and came about at the exact time mobile phones were blowing up, but I think the differentiating factor was the extreme ease of sign in, in combo with that same mechanism cutting spam made it an obvious choice. Just logging into software with username and password was a big UI hurdle back then for the older people in my family, and probably still is.
I specifically remember one use case that WhatsApp fixed and that google/apple/Microsoft had no answer for. Sharing contacts. It was so hard to share contacts with non tech literate people, and I do not know why all the other messengers by the big companies could not do it, but WhatsApp made it dead easy to send and receive properly formatted contact information.
Everyone around me loved it, though, so the joke was, once again, on me. How come almost nothing I like is popular. And vice versa
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.