Markers are good for marking where to increase or decrease, so you don't have to count each time.... patterns will say to knit within 2 stitches of the marker and then Inc. or Dec.. I also use markers when casting on, putting a marker every 20 COs or so... then I use a different color when i get to 100. Make it wasier to get back into it if I lose count.
I ALSO use the hangy-type stitch markers to hold a stitch I've dropped until I can knit around to it. They're good for marking rows, too, so you can follow directions like "INC 2 sts every 6 rows 4 times" without needing pencil and paper. AND if you do use pencil and paper, and LOSE the paper, you can easily see where you left off.
Finally, I use them when converting a flat knit pattern to a knit in the round. I use one marker for the beginning of the round, which I make the "center back" , and then I use 2 other differetn colored markers for where the side seams would be, so I know where to inc and dec.
You can also use them to mark cable or lace panels... say you want a cable panel of 20 stitches in the center of a seed stith scarf.... well, you just place a marker on the needles to mark the beginning and end of the cable panel, and you know where to follow the chart. I suppose you could use this methbod for a sweater that had multiple cable charts as well... chart A goes between the blue markers, chart B goes b/t the red markers, etc...
i love stitch markers.