Primers prep the skin for easy application of foundation and create a barrier between foundation and skin. Primers work to lengthen the wear of foundation and sometimes, even out the skin tone. Prepping the eye lids with primer is ideal for stay-put, long wear shadow. Primers reduce the chances of eye shadow creasing or sliding. You can also choose to prime lashes before applying mascara. Lash primers condition lashes and some lengthen or volumize as well.