There are some beauty products that seem to exist just to make multi billion pound companies richer, and with so much out there screaming amazing claims, it's hard to know when it's really worth buying.
However, one seemingly unnecessary product that I've been swearing by for a while now, is using an eyeshadow primer/base.
I regularly rebuy this one.
It's a Mac Paintpot in Soft Ochre, which is a small pot of a nude cream eyeshadow. I use it as the first product on my eyelids, every single day. I sweep it with a brush, all the way to the eyebrow. I'm pretty pale, and this nude is perfect for me, but Mac could suggest a better nude if you have a different skin tone.
It prevents eyeshadow creasing and smudging, as well as helping your eyeliner hold tight. It also works as a nice matte leveller for the brow bone, I often don't add eye shadow there, just leave the Paintpot on its own.