A. It depends on your skin. Some people benefit more from primer than others. Some can just use a moisturizer as a primer. A face primer will help hide imperfections in your skin so your foundation goes on smoother.
Older skin that has more imperfections than young skin benefits most from a face primer. Also, anyone who has acne scarring. A silicone based face primer puts a film over your skin. This film floats on top of of skin helping to hide large pores, shallow scars and small lines.
There are also tinted primers designed to conceal imperfections such as redness. Some primers provide oil control allowing foundation to look fresher longer. The foundation applied over any face primer gives you that flawless finish. BB & CC creams are great face primers.