A few of you might be thinking 'Ummm what's a primer?', so let me just put ya at ease before I continue yammering on. Primer is a makeup product used underneath another product in order to make it go on smoother and last longer. You can learn a lil' more about it in my beauty dictionary here if you want. The two main primers you come across are for eyes and foundation.
Now that we've got that outta the way, let me ask you a question. Are you happy with how your foundation goes on, and even more importantly, how well it stays on? If you can put a check next to both of those boxes then guess what...you don't need a primer! The same goes for your eyeshadow. If you are happy with the results you are getting using those products on their own you don't need to use a primer. If, however, you have noticed issues like your foundation fading quickly throughout the day, or your eyeshadow creasing and moving around, that is when you definitely want to invest. If you've never used one before, you will probably be surprised with the difference it makes.
Even though I said you don't need a primer if you're happy with the results you're getting from your makeup, in my personal opinion, everyone should try them at least once because they can offer a lot of benefits that make your makeup better, even if you don't have any specific 'issues' you are trying to avoid. I almost always use a little primer under my client's foundation to make sure it goes on extra smooth and lasts longer, and I definitely use something as a base under eye makeup, whether that's a primer or just a bit of concealer and powder to act as a primer (hot tip). Depending on the product you get, you can find foundation primers that offer a lot of bonuses, like filling in pores (try this one from Benefit), controlling shine (try this one from Hourglass), or giving your skin a glow (try this one from E.L.F.). And when it comes to your eyelids, which produce oil throughout the day, a primer (like my favorite from Urban Decay) will keep everything more matte and in place.
The bottom line is, as with most things in makeup, you don't absolutely have to rush to the nearest store and buy a primer right. this. second. BUT, if you wanna take your makeup game to the next level, it is something I'd highly recommend. At least browse the options out there the next time you're at the store, and see if anything tickles your fancy. It can't hurt.