"I´ve never needed a primer before, why would I need one now?" - I know that some of you have at one point asked yourself the same question.
A primer smoothens out the surface, making it easier to get a good reslut. The primer also "grips" on to the used products helping the makeup last all day. Depending on which primer you are using, it can help tightening pores and reduce shine.
Perhaps best of all, it works as a layer, or as a shield, between your skin care and makeup. Which lets your serums and day cream do it´s magic in peace.