I've seen dragonflies perch like this several times before, and I wondered why. So today I did some googling and found out this is a way to keep cool (and the pose itself even has a name).
By lifting their back end up in the air, dragonflies reduce the amount of surface area they are exposing to the sun. And holding their wings out also provides shade for their thorax.
But this is a male blue dasher, so there is one other reason he might be perched like this: to defend his territory. There was another dragonfly buzzing around nearby when I first saw him, so it's hard to say. Either way, I was happy he decided he liked this little branch enough to stay there a while.