I’m an expert in game design and economy design (+10 yr experience professionally).
You do this so that health doesn’t feel rare. The same thing with ammo. If you don’t drop ammo for weapons, even when the player is full, the player may believe ammo is rare, hoard it, and not shoot. So if you want to incent players taking risks, you drop health and ammo, even at full, so the player feels they can experiment.
This was noted in the GDC talk for Ghost of Tsushima: they do step on the drop rates when you’re low to give more than usual, but they don’t do the reverse (e.g. give you none at full) because they found, in play testing, players hoarding ghost tools (and therefore didn’t use them) unless the player believed a bunch was available.