There are all kinds of stitch markers.
I will often use one that’s unique as the beginning of the round marker. When knitting in the round, I need to know where the new row starts.
When doing colorwork, I mark each repeat so if I’ve missed a stitch, I know right off the bat and don’t knit all the way to the end of the row and find I have an extra stitch or I’m short a stitch. In the piece shown above, the orange circles mark the repeats. You may see a green marker also. There are two of those – one for each side (as in side seam but there are no side seams). I need to know where that is for shaping the arm holes.
The marker shown above is actually a slice of pizza on a little lobster clasp. They’re intended just to show you the progress you’ve made. They’re called progress keepers. In theory, you place them on your knitting when you start for the day and at the end of the day you can track your progress. I mostly don’t remember to use them.
The markers that I use the most are these removable plastic safety pin type markers. I use these for many purposes. If I see a dropped stitch, and can’t get to it right away, I’ll secure it with one of these. When knitting sleeves, I will mark the decreases so I know exactly where they are when I start the second sleeve. Also, if I’m counting stitches, I’ll put a marker every 10 rows so I don’t have to go back and count 40 or more rows at once.
As you might guess, a knitter needs a whole lot of stitch markers! 🙂