Apps, like WhatsApp, that provide VoIP, can be tremendously useful.
Traveling abroad, and using a local SIM? No need to worry about the local number, or voice minutes; it's all data, which is usually cheaper to buy, and refill.
VoIP is also not constrained by international boundaries, outside of some authoritarian countries.
Someone at home can easily reach you, without having to know that temporary local number, or initiate a costly international voice call. Free international calling, in any context.
Decide to split up, or get separated? Text or call.
It works pretty much any, and everywhere, doesn't cost extra, and is cheaper than making traditional voice calls, nor encumbered by a specific phone number.
What's not to like? Afraid Zuck is listening in? You may want to buy a bridge along with that notion that cell network switches are any more secure, or resistant to tapping. If anything, they're less so, and normal voice and SMS easily tapped. While not unbreakable, the better chat apps employ end-to-end encryption for their services.