More great behaviour photos Leachy
Michel367 is correct about getting minerals from the soil to add to those occurring in their diet, but this behaviour also has another purpose in that some of the vegetation that comprises their diet contain toxins, especially some of the leaves. The minerals counteract the toxicity and help them digest more easily.
I read somewhere that Kudu die off in certain areas if kept fenced in in certain habitats as they cannot move to new areas to browse on different leaves. If I come across where I read this again, I'll post more details.
Sprocky, I've only seen this in Pilanesburg as well. I have a photo of a Giraffe at the salt lick somewhere, I'll post it in a moment or two.....