It's to reinforce the heel flap, you can choose to do a regular stockinette stitch like the rest of the sock. Also, fyi, I have found that when doing a heel flap that the slip 1, knit 1 on 1 row and slip 1 and purl the remaining sts and cont. this 2 row repeat on the heel is enough reinforcement....IMHO. When I have done the sl 1, k1 on 1 row, then the sl 1, p1 across the row and continue it this pattern that it becomes too tight of a heel; but you do have a different look ... just my preference. Any help at all?!
Addicted to the Knit is Becka
"I can do everything through Him who gives me strength."
Slipping the first stitch on the heel flap gives you a nice chain stitch on each edge of the heel flap to use for picking up the gusset stitches. If you don't slip the first stitch, picking up the gusset stitches would be a real PITA.