Some will tell you that primer is not necessary to create strong PVC joints. Others swear by it and slather it on like their lives depend on it. While you may be able to create strong joints without it, primer is an important part in making sure your joints are as strong as they possibly can be.
We recommend you use primer on any joints that will have liquid media passing through them.
The purpose of primer is to begin the chemical reaction that softens PVC and to provide an even prepped surface for the solvent cement. As you may know, PVC joints are not glued together exactly, but fused through a chemical change. The softer the outer layer of your PVC, the better it will cement in your joint. This is how primer helps your joint strength.
In many states plumbing inspectors will look for primer and only pass you if they see liberal use on all joints. Most primers are a vibrant purple color so that its use is evident, but there are also clear primers. If you're working on an inspected job, find out if that state requires the use of primer. If it does, choose a brand that offers a colored primer that will be easy to see, and use it liberally beyond where the pipe will meet the socket. If the inspector can't see it, there's no point in using it.
When it comes time to apply your primer make sure you are 100% ready to prime AND cement. Primer is effective only when wet, so you need to move quickly once you begin. Usually PVC jobs require about half the amount of primer as they do cement.
In short, primer is your friend. It may not be completely necessary on all jobs, but we recommend you take all the precautions available to ensure your joints are leak-free and as strong as possible.