Why ant have king and queen?

i don't really know bout that. But I've seen 2 big (black) ants move together side by side followed by 2 long lines of small ants behind them. The big ant on the left side has a big head while the other one has a big abdomen with gold stripes on it. I assumed the big head as the King while the big abdomen as the Queen of ants, and the small black ants as the "soldiers".