If I make some changes using, for example, the Curve tool, I want to be able to quickly see the difference between (a) how things look now and (b) how things looked before I adjusted the curve. But there's no way to do this in Lightroom Mobile on Android.
Of course, pressing and holding the screen doesn't help here because this shows the state of the image before all edits, whereas I want to see the state of the image before the current edit only. The Undo feature also doesn't help, because this undoes each curve adjustment individually. I think the only workaround is to use Versions, which is far too tedious and time-consuming.
This is such an important feature that I am surprised it hasn't been implemented yet. Its absence makes Lightroom Mobile almost unusable for me, even though I pay for it. Snapseed already has this feature. Note that my request isn't restricted to the Curves tool—ideally, this before/after button would be available for every tool.