Got a project that involves, among other things, feeding a more or less constant current, on the order of 2-3 mA, to some components, while both their voltage drop and the supply voltage may vary.
Back when I were a wee lad, there were things called "current regulator diodes," which I never had occasion to use.
And, look! They still exist. And are available in SMT. And currents in the range I want. And...
Oh. Minimum forward voltage to ensure supplying that much current is ~2V. My supply headroom is, worst case, rather less than 1V.
So, if in fact we end up needing current regulation (a subject which is still much open to question), a CRD won't do it. There are other ways, but they take up more board space.
Ah, well. Knowing they still exist is useful; I occasionally need a small constant-current source in an application where a CRD would work.