In many countries SMS can get expensive, especially for international use, depending on the phone billing policy. In poorer countries people often use free wifi

In many countries SMS can get expensive, especially for international use, depending on the phone billing policy. In poorer countries people often use free wifi areas for data-based messaging. Hence the popularity of WhatsApp. Even if you have a limited mobile data plan, WhatsApp uses the capacity very efficiently, so it's still affordable.