When should you apply a primer coat before painting?

It depends on the condition and colour of the paint on your wall. If it's all scuffed-up in places, you probably want to prime those spots to make it more even (after you've ripped/peeled off the pieces that are sticking up of course).

Also, if you're painting a light colour on top of a dark colour, it might be a good idea to put a layer of white primer in the middle.