It seems like the only point is to get around the few remaining billing plans on the planet that don't have unlimited text messaging.
This comes up every single time something is posted on Slashdot about WhatsApp.
Lots of people have packages with tonnes of text messages making them, essentially, free or very low cost - however SMS doesn't do anything beyond 1:1 communication in plain old text. So picture sharing and group chats are out.
MMS can do that, but it's often excluded from SMS packages - so after a few messages it can start to get rather expensive. Even more so when you are sending these things to different countries.
iMessage can do that too and it's nicely integrated into iOS. If your friends aren't using iOS though then it all falls down.
So, combining these all together gets you the following wish list:
- Very cheap almost to the point of being free.
- 1:1 and group chat support.
- Picture and content sharing.
- No additional fees for sending worldwide.
- No additional fees when you're roaming.
- Not tied to users of one operating system.
WhatsApp (and the like) fill this gap.