That dolmens were built before the stone circles and maybe also most stone alignments may have to do with building most needed things first.
The people who constructed these things thought maybe that it was very important to be able to spend the night in safety, also from lightning and intentionally caused fire. Once these buildings were there, they could be used and complemented with further defensive constructions such as stone alignments.
The main Carnac stone alignments make me think somehow of the Great Wall of China but in a much earlier period with much less advanced technology and of course on a smaller scale.
P.S. I read somewhere that even the Romans camped near or in the Carnac stone alignments. If that is true, they must have found the place apt for that. They were rather experienced in military affairs ....