The on-camera flash will trigger the Speedlights if they are set to be slaves (unless the area is very brightly lit, it may not). So will the external trigger but you won't have to fire your cameras flash & it may have a greater range (don't know why you would need that though...shooting merchandise is not that complicated).
K - Could you clarify? I'm a bit confused by this...I bought a speedlite thinking I could useit as a slave but it does not have a photosensitive trigger. After reading the manualseveral times it seems to me that speedlites can "talk" to each other with some sort of radio signal, but my on-camera 20d flash cannot communicate with the 580ex. Orcan it??? This flash is so complicated when you take it out of "P" mode off camera LOL