Primers are as diverse as foundations now. They come in hydrating formulas, matte formulas, and can give you a glow or provide oil control. A good primer can set your makeup apart from amateur work. Proper skin prep should have revealed the client's skin needs. If you or a client has oily skin then a mattifying primer is best; alternatively if you or your client's skin is very dry, a hydrating water or silica based primer would be better to lock in moisture on the skin.
Using one primer isn't always ideal. When starting a face, ask the client what their problem areas are. You can mix and match primers as needed. For example, if a person has an oily tzone but is generally dry everywhere else then apply the appropriate primer to the appropriate area taking care to match formulations as much as you can. Commodity products such as M.A.C's strobe cream, pore filling primers or color correcting primers can be layered underneath or above another base primer so that the makeup's results are flawless.
Primers usually come in two formulations: Water or Silica. Silica works best with oil based and silica foundations whereas a water based primer can work with any foundation but you must allow it time to sink into the skin. Just like with skin prep, I advise that your application routine fits in time to let products settle into the face before applying complexion products.