There's one bloke that's visited a lot of bullauns in Gottland. His theory is that bullauns (and they're always single in Gottland) were for holding the bases of tree trunks that formed the shafts of prehistoric windmills.
I'm assuming bullauns are often near springs. If that's the case, obviously this would be so the windmills can pump water using prehistoric plumbing techinques, probably involving very large reeds, which would be connected to prehistoric mixer taps in the narest fullacht fiadh.