What are stitch markers used for?
Knitting Stitch Markers
Knitting stitch markers are used to mark increase/decrease points in knitting patterns, especially for use in making sweaters, shawls, and toys.
Colourful Skulls Stitch Marker Set found here
Ridge and Furrow Rainbow Triangular Shawl pattern on Ravelry
They mark the beginning of the round when knitting any garment in the round like mitts, socks, sweaters, hats, infinity scarves and toys.
Purple Agate Stitch Markers found here
So Faded sweater pattern on Ravelry
Experienced knitters like to thread small ring markers between large groups of cast-on stitches to help keep the stitch count correct. This example is a shawl that requires 250 stitches. The ring markers separate the cast-on stitches into groups of 25. You would remove the ring markers when beginning to knit.
Small Ring Stitch Markers found here
Stitch markers are used to mark the beginning and end of repeat lace patterns so the knitter can review the lace stitches for errors before beginning the next pattern repeat.
Crochet Locking Stitch Markers
Crochet locking stitch markers are used to lock onto a chain stitch in crochet to mark the beginning of the row.
Multi Colour Flower Bead Stitch Holders found here
They can be attached to dropped stitches in knitting, so the dropped stitch doesn't unravel any further before it can be fixed.
Rainbow Crochet Stitch Markers found here
They are used to mark the cross-over row when knitting cable patterns, to make it easy to remember how many more rows are knitted before crossing the cable again.
Double Fantasy cable pattern in Vogue Knitting Stitchionary 2