Apple added a secret button to your iPhone, and you may not have even noticed
Anybody use this as a page turner in reader apps?
I knew about this but didn’t think it would be useful for me.

And having just tried it out (for scrolling) I find it rather awkward: tapping the back with enough force to trigger the back tap inevitably shakes the screen and it seems too disruptive to eye tracking. And the default scroll doesn’t scroll page fully. Maybe with practice it would be smoother but I’d be worried about frequent repetition and how my fingers would hold up.

I didn’t see any built in actions that correspond to page turns, and I do not see any way to add these gestures (some reading apps allow mapping volume buttons to page turn but not the ones I use).

By contrast, Voice Control works very well and is totally touch free, but of course it’s only good if there’s nobody around to be disturbed by your commands. The built-in gestures are usually sufficient to do page turns and scrolling, and you can add other gestures as needed (e.g. to scroll fully, or to navigate apps that aren’t fully instrumented for Accessibility). And this also works on iPad.
Not for me, I use a case on my SE2020...
It does not matter if you use a case. It’s an accelerometer trick.
I tried it out long ago on my iPhone XR with an Otterbox Defender case, but even though it worked with the case the feature itself was rather flaky as I recall. I did not find it useful enough to keep using it. BTW, it is really not that secret of a feature for anyone who reads the manual or watches Youtube videos on iOS new features, however Apple didn't go out of their way to publicize it.
I should think that Otterbox case would add so much mass that it would become more difficult to apply enough force to accurately double and triple tap.

Still, it’s very hard for me to come up with a use case for this feature.
The case made it difficult but it did work. However, like you I simply couldn’t find a useful scenario for this feature.
I was interested to see that you can have it bring up Spotlight even with an app in foreground. This at least is one gesture instead of two (home button or swipe up from bottom to get to home, then swipe down to invoke Spotlight). But that’s the best use case I’ve come up with so far. For the moment that’s what I have it set to do: I use Spotlight a lot.

I am also trying out App Switcher. That seems slightly better than the slow swipe up (with FaceID phone).

A little surprising you can’t Unlock with back tap (with FaceID phone).
Indeed. I’ve just checked it out and there really isnt anything I would use it for. So no. Still not for me even though it works with my spigen case