A Place of Worship
This is by far the most common belief regarding the dolmens. The locals believe to this day that the ground is sacred, (see The Local Beliefs).
Bronze age people would gather in small groups to pray to their gods, sacrifice an animal and preform rituals. The dolmen would be in the center, around it they would gather. Smoke or light would, maybe, come out of the round hole as part of the ceremony. The "wise-man" might say a few verses, the birds would stop tweeting for a brief moment as the animal is killed, the smell of blood roasting on fire would fill the air and the people would each offer their own personal prayer.
This theory can explain the use of huge blocks, the hard work involved, simply, as worth it. the impressive, long during place of worship is worth the work needed to build it.
My main problem with this theory lay in numbers. A place of worship, I imagine, there should be one, or one for each village or community. In effect, there are dozens of dolmens, some isolated and some clustered in groups of 4 to 12 dolmens. It seems unlikely to me that bronze age people would need so many places of worship in one location. I can imagine the dolmens being an actual tool in the ceremony, if there are many people at a time, they might preform the ceremony in the same location but in smaller groups, each having it's own dolmen. This idea, however, requires that the making of a dolmen is relatively easy. I can imagine an entire community putting everything they've got into building one of these dolmens as their place of worship. Cutting and transporting the stones, shaping them, cutting slots into them and assembling the structure. that they made more than one in the same location indicates to me that the making of one dolmen might not be as difficult as I imagine it. They might have a way, a method, a technology that makes it easy, or easier.