Well it need not be "traditional." But for humans holding hands is very symbolic. I don't hold hands with anyone and I only do it in certain contexts. Holding hands is something many romantic partners do in the West, and they typically do it on dates or walks together etc. This is not the same for all couples cross-culturally. Holding hands is not the universal way couples walk together. It is something that we socially learn. It is a part of western culture. Likewise, there are certain chimp behaviours that are involved in friendship and courtship that are learned group-specific behaviours. Some will have certain hand clasps, grooming practices, or hand gestures that are only done with certain individuals in certain contexts. They represent cultural variants that can be mapped across chimpanzee populations in sub-Saharan Africa.