I’ve looked at all of them on you’re site. Does the fact they’re so round suggests more of a quern stone function than pestle and mortar. The outflow groves might fit with this – wouldn’t the ground-up material accumulate in a convenient pile in those? Having a groove connecting to another depression also looks convenient.
I’m struck by another bit of convenience – some of the earthfast ones look as if there’s a space deliberately left on the rock surface for someone to sit at the side while they’re working, and the Deer Stone is the classic example http://www.megalithomania.com/show_site.php?site_id=300&image=734
… absolutely inviting two people to spend an hour taking turns there – and that one has the outflow as well. The multiples could also suggest a communal grind (I’ve been trying to avoid that word).
The fact that in multiples all the holes seem carefully placed on the same plane is suggestive of an intention for them to hold liquid, but communal use for grinding would fit just as well.
The placement near wells or springs is also consistent with these thoughts, a village gathering place and function area. I wonder if there are flat clothes-washing stones to be found as well? I grew up in a semi-Neolithic village with a communal water pump and a big sandstone butt underneath that we sat on. That place was the centre of our social life…