It's free. No length limit. You can send pics and vids. No international fees.
It's cheaper (free). All this value stems from carriers's infinite greediness in making us pay (international) SMS and MMS.
It's only free if you have a data plan, right? (or connected to WiFi). Historically in NZ, SMS has been basically free, unlimited but data has been 10mb, 100mb etc.
We're now trying to catch up on the world with our data plans on 3G and 4G networks, but even then it's about $50 NZD for 1gb data.
It is broadly speaking lower cost, allows media sharing (photos/sound clips) and provides delivery receipts.