It started with the iPhone SE being able to use cheaper iPone 5 & 5s cases, then along came the 5th gen iPad 9.7-inch. It had the same dimensions as the original iPad Air, so iPad Air 1 cases worked with it.
The new iPhone SE is the same size in all three dimensions as the now-discontinued iPhone 8. Ergo, I go to the local office supply store when my new SE arrives (I'll order one tomorrow) & buy a $15 iPhone 8 case instead of a $40 SE (2) case.