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!