Barrows can be both symbolic and  physically existent and were used to promote spiritual experience - via sensory deprivation and one suspects also fear and hypoxia. 

Symbolically a barrow has the same meaning as a 'cave' being both an entrance to the spiritual world and a means of 'doing the dive'.

It is worth noting that the symbolic meaning and the physical reality are not at odds, it is more than likely that a person going out of body by being incarcerated in a barrow for some length of time would indeed 'do the dive'.

One added point of interest about barrows.   Alfred Watkins  was perplexed at the number of barrows  when he did his research on ley lines, more than was needed to mark ley lines or for that matter bury people, rather indicating some were used for just spiritual experience - although some could have been both, there is nothing more likely to promote an out of body experience than being buried alive with your dead relatives.

Watkins then goes on to mention other examples in Dorset, Wiltshire and the Yorkshire Wolds where the same observation holds true – very many barrows all aligned, but far more than is needed to mark a physical track.

If however, ley lines are regarded as spiritual rather than physical paths, his observation can be seen in a different light.  It makes sense for any prehistoric people, when burying their dead to place the barrow on the spirit line.  In this way they ensure that the soul has an easier flight to the spirit world and it also makes sense if these are used for spiritual experience to line them up on a known 'line of access'.