The standard opinion given out nowadays is that they were used for storing foodstuffs but there are in fact several theories competing equally in the archaeological community including, cough, ritual. Plus they shade off into wells and settlements at the edges. Also there appear to be two types according to when they were built.
The later kind found in regions such as Ireland and (IIRC) the Western Isles tend to date from Viking to mediaeval times. These seem to be more obviously defensive, many being built into places such as streambanks and cliff-faces (though you could argue that these places also tend to be nice and cool, by water and/or wind, and hence suited to becoming storehouses also).
As to the earlier kind they would appear like burnt mounds to have had several uses, not always exclusive, and have been built both from and into other structures [I wonder if some originally stood alone before later incorporation, and think this applies even more probably to 'wells'].
Though this is a weekend, and a Bank Holiday one at that, I expect others here will bring their minds to bear on this subject - that's my excuse anyway.