You've been doing the spider curl for a few weeks now, but do you realize why that exercise was chosen in this upper-body circuit?
By pressing your triceps (the back of your arm) against a pad, you lock your elbows in one space. Often when you do biceps curls—say with dumbbells in front of the mirror—your elbows can move all over the place. When your elbows move, your shoulders—and often momentum from a swinging motion—will do the work instead of your biceps.
The spider curl refuses to allow such wasted movement. It locks your elbow in place and forces you to curl the weight with the two muscles of your biceps. That pointed focus helps the target muscles grow.