When I first began knitting, I wondered why I would need a bunch of stitch markers. All I was knitting was simple scarves and hats. I used one marker on a hat knit in the round to mark the beginning of the circle.
As I began to knit more intricate projects, I realized why I would need bunches of markers.
Any time a pattern will repeat itself, placing markers to section off the repeat is helpful. You can see this in the knitting basket photo of mine below where I was knitting a shawl. Marking each repeat helped me keep track of the stitches so mistakes could be caught quickly.
Color changes may require markers, as will increases and decreases.
In other words, there are many times within any one project where I will need to be reminded of something... and a stitch marker does that.
Each project will probably require it's own number of markers. Does any knitter ever have only one project going at a time?
Hence the need for bunches of markers.