You need these when the Damsil nymphs are coming ashore in the height of summer. Trout are cruising the shoreline looking for the damsils as they swim from the weed beds to the shore and emerge where they climb up a stem of grass and shuck their nymph skin turning into bright red and blue damsil flies.
This bead head version fly can also be deadly cast towards the shore in a weedy lake and stripped back towards the boat at a slow to medium speed.
Note: Image does not show the bead head..