I have noticed that heel flaps constructed with K1, S1 repeat seam to be rather common. I know it looks nice and different (especially with multicolor yarn) but is there an attribute that lends itself to the heel flap? Does it pull a bit tighter, with the slips and all, or is it a bit more durable in the shoe because of the added thickness? Just curious.
ThanX for your input.
-me
Maybe because it seems thicker and more cushiony than the rest of the sock.
The heel gets a lot of wear, so it is made thicker. Makes it more durable, like you said. 
Cool Cool
ThanX fro the prompt replies.
3 reasons:
1 the slip stitch pattern changes the gauge of the knitting, (row gauge) -it packs more rows into the same place
2–because the knitting is denser (see #1) its is stronger
–and because of the slipped stitches, the rows don’t line up either… so a single break is less likely to spread (it might run, but it won’t run and expand right to left (because the stitches either side were knit in a different row!)
3–and when that single thread breaks, there is a 50% chance there is a thread behind it… to still provide cushioning.
these 2 factors, denser (and thicker) fabric, that reisists becoming a hole if the yarn wears through and breaks, and that provides cushioning in the case of a break… make a slip stitch pattern a good choice for any place where there is hard wear!
Thank You, Thank You!