1, In my country that were the first (or almost the first) that allowed chatting not part of your SMS package that was costly back in the days 2. You just need a phone number, no other social or email login required 3. It was one of the firsts to allow the groups, including admin rights 4. Your contacts are imported to the app OOTB 5. They have the broadcast option 6. Last seen, when message delivered/read is something everybody wants to know