Face primer provides a surface for your foundation to stick to and a barrier to keep your makeup from sinking into your pores. Your makeup goes on more smoothly and looks flawless. Primer makes makeup last longer because it doesn't slide off. Primers come in different forms for oily, dry or combination skin. Choose the one that matches your skin type. If your skin is combo and you have trouble keeping your makeup in place over the day, go for an oil control formulation. I put my sunscreen on first (a big necessity, you don't want to get sun damage!!), then primer, then foundation.
I personally do not think that primer is a necessity--it simply is just a luxury (only if you find the right one)! You do not need to spend extra money on a primer if you are already happy with your makeup! However, if your face tends to get oily, you have large pores, or your makeup tends to wear off faster than you want it too; then by all means I would recommend it! I have tried a lot of primers and I have noticed that they do not really do much. I mainly bought them because I was concerned about my large pores, and I wanted to minimize them, however, the primer made my pores more noticeable than they actually were, so now I completely stopped wearing primer--and I'm happy with my makeup look! Nevertheless, if a good primer comes along, yes I may start wearing it again, but until then I'm happy!
I hope this helps!! )