I would guess to keep you warm, to protect from irritants, and to protect from the sun. If men were the hunters they were out in the elements more.
White people had less protection from the sun (less melanin in their skin, which is a natural spf) so maybe hair is what helped to protect them. Men in the area of the sunny Mediterranean have the most body hair. Also, people with anorexia sometimes grow body hair, so I am guessing that is because they don’t have a fat layer to protect from the cold.
Natural selection might be another reason, maybe men preferred women with less body hair and so that trait was promoted. Women with PCOS often have more body hair and don’t ovulate. It’s not really cut and dry though. They sometimes pop out an egg, and they don’t always have a lot of body hair. My guess is there was a lot fewer cases of PCOS in caveman days since the diet would have been so different. There is a connection between blood sugar levels and PCOS.
I think pheromones and scent was part of the reasons also for hair.
We have better shelter now than a few thousand years ago and more access to clothing and sex is more scheduled so I don’t think it is a big deal that we remove body hair.