Most primers on this site come with caveats. This one largely does not, and the reason is anatomical rather than cosmetic. The upper lid is the thinnest skin on the body, it sits on a muscle that moves every time you blink, and as the skin above it relaxes with age the lid becomes hooded — meaning the mobile lid folds in on itself and there is a defined crease for anything loose to collect in.
So eyeshadow on a mature lid has a destination, and it finds it quickly. Powder shadow applied at 8am is frequently a visible line in the fold by 9.30, with the lid itself looking bare. That is not a pigment quality problem and buying better shadow does not fix it. The shadow is simply sliding, and the reason it can slide is that there is nothing tacky underneath holding it.
An eye base solves it directly: a thin tacky film that the powder grips, so it stays where you put it. This is the one case in the whole primer category where the mechanism is straightforward, the effect is large, and the effect lasts all day rather than three hours.
The main thing to get right is quantity, and it is a different scale from face products. Less than a grain of rice for the whole lid, sheered out to almost nothing with a fingertip. Too much eye base is worse than none: a thick tacky layer grabs the first touch of shadow and refuses to let you blend it, so you get a patch of intense color you cannot move — and on crepey lid skin a thick film also creases visibly on its own.
If your lids are also oily — which plenty of mature lids are — that is the other half of the problem, and a base matters more rather than less. Oil is a solvent for the binders in powder shadow, so an oily lid breaks shadow down and then provides a channel for it to run into.
