[rkomorn]

For me, mainly: no international cost, no metered cost (other than data), no extra app like WhatsApp to install (but other party needs iOS).

Edit: that said, nowadays, maybe because I'm back in the EU, I use WhatsApp way more often than iMessage.

[BenjiWiebe]

It is easier, however, to install WhatsApp then to switch from Android to iOS.

[rkomorn]

Of course. iMessage is just a convenience when someone else has an iPhone.