When you have a deep shot near the edge of a panel, you'll want to put the center of the tab RIGHT IN THE CENTER OF THE DENT. However, using a round tab puts the edge of the tab OVER the edge of the panel. When the edge of the tab hangs over the edge, it loses adhesion the moment you start a pull. 

The triangle shape allows you to place the PERFECT size tab RIGHT AT THE EDGE OF THE PANEL WITHOUT HANGING OFF.  Now you're centered on the dent and ready for a clean, strong pull.