[marcandre]

Or from any browser: https://wa.me/<phone number, just the digits, starting with country code>

[j1elo]

In fact that's simply what the app does. It's just a handy way to have it on the phone as a shortcut and not having to remember the details of how to do it. Although with the sibling comments about Whatsapp itself allowing to do all this, the helper app seems less useful now.