I frequently use the selection, but I don't like the implementation of it. Rather than the selection being between the starting "anchor" point and your moving cursor's position, the cursor you're moving around just creates an ever-growing selection that can't be subtracted from. So if you're e.g. selecting across multiple lines and you bring the cursor up to the line above, it'll select everything from the starting point to where the cursor is now, but if you bring it back down it doesn't unselect that text. So if you accidentally select something you didn't want to, the selection is essentially ruined and you need to start over. I have no idea why they implemented it like this.
Selection was introduced at the same time as “space bar cursor”, which is with the introduction of 3D Touch in the iPhone 6s.
Actually, it wasn’t even a “space bar cursor”, as the way of triggering it was with force pressing and not by holding the space bar. It works much better with 3D Touch, as you can move the cursor around and press harder without lifting to start selecting, then release a bit to stop. My iPhone 8 can still do it, and I’m really going to miss it when I upgrade.