I was perfectly happy to use SMS and voice calls over the PSTN, but I found that many people in the US have phone numbers that can't receive (or in some cases send) calls or texts to foreign numbers. I don't have unlimited texts or calls on my phone, so I like the fact that I can use my home internet or public Wi-Fi to call at essentially no cost. I've had regular conversations with people in the US that I probably wouldn't have had without WhatsApp. When I'm in the US, it's a useful adjunct to using roaming on my phone - my phone plan makes texts and voice calls there expensive and data insanely prohibitive, so Wi-Fi and WhatsApp are great for me. I'm not familiar with the other apps referenced here.