From my observation, they were sitting there like waiting for the others to be done taking pics so they can replace them for their turn. Some (without the dresses) just really like sitting there and watch the others, literally like how you do in a park... So yeah...
Based on the other comments it may be overcrowded throughout the day haha since this temple is in the middle of Bangkok, easily accessible, and quite small for the amount of tourists.
Oh, that sucks. I guess the best bet is to go during the rainy season for less tourists, and hope it isn't raining when you go.