Do I need to prime every time that I want to freshen my walls with another coat of paint?

No. Primers are not always needed when repainting. If you are repainting a similar colour, you can lightly scuff sand to remove the gloss and clean the surface. Once that is completed, you can simply repaint over the existing paint coating. More than one coat may be needed for uniformity. Please note that switching sheens or drastic colour changes may require a primer to minimize the amount of topcoats used.