The voice recording was the first feature, and people started using Whats App as a free voice calling app.
After a few months from this update, Facebook decided to add a video feature, making this app a great group video chat app.
This app is compatible with both Android and i OS, and it even works on slower connections like the 2G one. This app is absolutely free to use, with no restrictions whatsoever.
Still, Imo often displays some ads, but they are not bothering and you will get used to them.
Many people use this app, strictly because it's linked to their Google account.