For me it’s the app drawer, moving the typing cursor and previews (notifications, messages etc).
But you have to press-and-hold, since there's no force press.
On my new iPhone XR it's far more convenient to use, since the spacebar is not at the bottom, and scrolling down (e.g. in order to edit a message) works more reliable.
It still doesn't feel as good as 3d touch on the old thing, e.g. it sometimes doesn't go into touchpad mode when pressed directly after a word and doesn't feel as precise. But it's good enough that I don't terrible miss it.
It is the natural and best way to do so. You press hard and the cursor moves freely like a mouse pointer, put it where it should be and release. Along with haptic feedback, it's a must have once you start using it. Really beats the magnifying glass method.
I would have been useful for devices like the iPhone SE too.