Eyelid/eye shadow primer does three main things. 1)It makes eye shadow colors apply more vibrant and truer to the color in the pan 2) It makes eye shadow last longer without fading. 3)It prevents creasing. The ELF eyelid primer does this for only 1$. I haven't tried many eye shadow primers, so I can't really compare the ELF primer to other primers.
The ELF primer does make eye shadows apply truer to the color in the pan. I applied several shadow swatches with and without primer. I applied each shadow in sided by side swatches. The left swatch is without primer and the right swatch is over the ELF primer. As you can see, shadows look much better over the primer. I took the pictures in direct sunlight, but I probably should have taken them in the shade. I apologize for the poor quality.
You only need to watch the video up to the part where I apply the primer. The rest of the video has most of the same info in this post with some random chatter thrown in. Less is more with primer. A thicker coat, just means there is more stuff on your eye to crease.