The directions are just being overly cautious. When you purl you bring the yarn to the front to make the stitch. When you are told to slip purlwise, you should automatically do the same i.e. bring the yarn to the front, then slip the next st without actually purling it.
Sometimes, a pattern will tell you to slip a stitch purlwise with the yarn in back (wyib), which would be different than a normal purl stitch is made.
The directions are just reminding you to slip purlwise in the standard manner i.e. wyif.