Stitch Markers can be used for for multiple reasons. A few of them include marking the beginning of a round or pattern repeat, mark decreases, increases or even mistakes. You can also view all Knitter's Pride MIO Stitch Marker Rings.
The large size are enormous - for size 15+ needles. Way to big for everyday knitting. Also, the markers feel cheap compared with other brands I've tried. They were inexpensive and were worth trying, but I don't think I would buy them again.