I can't see why messaging on whatsapp or any other app would make messaging any more comfortable or natural, or make the conversation "flow better".
I do, but then I actually use WhatsApp on a daily basis, like everyone else over here. Even my mother of 83. It's as normal to have and use as a phone.
It's more personal, you chat 1 on 1, although you can also have group chats, you can send photos with ease, take a photo & send it right away, or do face calls, send voice messages etc.
It's like making a phone call. You can actually make a phone call via WhatsApp as well.
And with that it's entirely different from a dating site.
But since it's more personal -to me anyways- I don't add anyone to it just like that. It's family and friends and if I have a partner, him too of course.
Maybe that's why scammers try to get you to it? --> Emotionally, you feel more comfortable and are more open. As a result, you can also fall for things more easily.