We can see from the diagram that the shape of urticating hairs and barbs are different. Urticating hairs do not appear completely on the opisthosoma at birth. We must not confuse urticating hairs with abdominal hairs. Urticating hairs do not cover the entire opisthosoma. The function of abdominal hairs is unknown.
Tarantulas also use urticating hairs to establish their territories, and also for making the cocoon.
It is possible that this last use is a way of protecting the cocoon. Except for Grammostola, the other genera have hairs of type III, and we can also add Brachypelma smithii. It would be interesting to know what type of hairs these species have. Among Avicularia, they very rarely throw hairs, except sometimes Avicularia versicolor notably when she has a cocoon or just in post-moult. On the other hand, Avicularia metallica cover their lair with urticating hairs, and when you put your hand in you end up with it full of these hairs.