I get charged a fee per SMS, but I just need an internet connection with WhatsApp. Even when I'm abroad and I switch my SIM card to a different provider contacts can still reach me.
I can have group chats in WhatsApp and send video, images and sound. Plus I like how it lets me know if messages have been delivered. It's also fast and doesn't drain my battery mercilessly. The times I've tried Skype on my mobile device I can see my battery meter go down as I type.
But mostly I use it because all of the people I generally interact with in real life are on it. All of my family, friends and acquaintances use it.