The other reasons given make sense to me, but I bet there is also some psychological benefit in having a regularly scheduled escape from home, and having a guilt-free excuse for it built in, which partly compensates for being forced to come in a few days a week. The contrast makes it easier to appreciate the company of your spouse and probably makes child-rearing seem less oppressive. People theoretically could manage this without work imposing it on them, but in practice, having to make and justify the choice creates stress.