Hydrating primer is the most sensible-sounding product in this aisle and also the one most likely to duplicate something you already own. The job it does — putting a layer of moisture between thirsty skin and your foundation, so the foundation does not dry out and go patchy — is exactly what a good moisturizer does. So the question is never 'is hydration good for mature skin' (it is) but 'does this product do something my moisturizer does not'.
Where the useful ones differ is formulation balance. A hydrating primer needs two things working together: a humectant (glycerin, hyaluronic acid, sodium hyaluronate) that attracts and holds water, and an emollient or occlusive (squalane, an oil, a fatty alcohol, a silicone) that stops that water evaporating straight back off. Plenty of products ship the first without the second, because humectants are cheap and photograph well on an ingredient list.
That matters more on mature skin than on younger skin, and here is why. A humectant pulls water toward the skin surface. If there is nothing holding it there, in a dry room or cold weather it can draw moisture out of the deeper layers and let it evaporate — leaving the surface feeling tighter than before you applied anything. On skin with a thinned barrier that effect is more pronounced, which is why hyaluronic-acid-only products so often disappoint exactly the people they are marketed to.
The second thing hydration does, which gets less attention than it deserves, is make fine lines read less sharply. A well-hydrated line is shallower and the skin either side is plumper, so the shadow in it is less defined. That is a bigger effect than most blurring primers achieve, and it lasts longer — which is why the fine lines page puts hydration above blurring.
