Hi! When you apply primer before under foundation, your skin will look smoother as it helps to control oil, seal pores and soften skin. Some primers have SPF in it which helps to protect you skin from the sun. Many people also use primer to help their makeup stay on longer. It is good to use a primer if you are going out for a long time with makeup on and you don't want it to smudge.
Yes, I do use a primer. I started using it this year, I use it everyday after I discovered it because I love what it does to my skin! My skin type is combination, so my T-zone often gets oily and breakouts easily especially after using foundation. so I actually stopped using foundation before I discovered primers.
I would recommend Hourglass Veil Mineral Primer, it has always been my favourite! And it really helps in controlling oil secretion on my T-zone and helps my makeup stay on. It is a product I would definitely repurchase after I finish using it. You can probably get a sample from Mecca to see if it is suitable for your skin. Hope I helped in answering your question!