I am no anthropologist. A quick search on the topic (not specific to those people) suggests decoration, identity, and "perception of how to live life" are possible reasons.
> Gilligan points out that humans were probably decorating themselves long before clothes even existed. "When you look at contemporary hunter-gatherers who don't use clothing, they decorate themselves brilliantly with body painting. You don't need clothing in order to do that."
> a hunter, before he tracks his prey, must cut himself and rub the cuts with a root called ssho /oa. And he must rub his body with it and wear ssho /oa in a band around his shoulders. This is to get the game to “run foolishly”, not knowing that it is afraid, and to approach the hunter as an equal. ...
> Far from being naked, or nearly naked, the Bushmen of colonial southern Africa had a complex and meaningful practice of dress. It was intimately related to subsistence, identity and their perception of how to live life in the world as they knew it.
http://www.bbc.com/earth/story/20160919-the-real-origin-of-c... comments that it might be decoration.
> Gilligan points out that humans were probably decorating themselves long before clothes even existed. "When you look at contemporary hunter-gatherers who don't use clothing, they decorate themselves brilliantly with body painting. You don't need clothing in order to do that."
https://www.thesouthafrican.com/lifestyle/debunking-colonial... points to several "contexts and situations where elements of dress [broadly speaking] seem to have been of particular relevance." One example is:
> a hunter, before he tracks his prey, must cut himself and rub the cuts with a root called ssho /oa. And he must rub his body with it and wear ssho /oa in a band around his shoulders. This is to get the game to “run foolishly”, not knowing that it is afraid, and to approach the hunter as an equal. ...
> Far from being naked, or nearly naked, the Bushmen of colonial southern Africa had a complex and meaningful practice of dress. It was intimately related to subsistence, identity and their perception of how to live life in the world as they knew it.
The author's book is at http://witspress.co.za/catalogue/dress-as-social-relations/ .