Sixteen feet across and three feet deep? Hmmm... Can't think of anything right off that could be used for. Maybe a water feature of some kind?
Swimming pools? From what I remember, it's fairly close to an old river.
As for Stonehenge itself, along with the other standing stone and earth monuments in England, I just figure they did it because is was
something to
do. Once your hunting or farming is done, what are you going to do, just sit around and stare at each other for months at a time? I remember as
a kid I was always with a bunch of kids digging holes in the dirt and building forts and things. Human beings just do that. Except these were built
by grown-ups who had bigger ideas and were a lot more familiar with astronomical events.
I imagine that over the years they just kept adding to it and subtracting from it and once in a while they'd have a party there where everybody would
dance around and drink and have a good time. Maybe go swimming.