It's supposedly an evolutionary leftover.
If women all menstruate, ovulate and potentially lactate around the same time, should something happen to one of them, another woman can take care of the babies. This happens with animals sometimes. You'd think what's the point of all the females in a group being in heat when only the alpha female is allowed to mate? If she dies or isn't around someone else can babysit, though some species will resort to infanticide and just have their own litter instead.
The synchronised periods is caused when women routinely pick up on eachother's pheromones. They don't even have to be living together. I read about a study once where all it took was for the women to smell eachother's sweat on a rag and it worked.
You're right. Women can make no sense at all sometimes. I hate it when they can be contrary, bitchy (towards eachother and men) and get angry when they're not being bought things/there is not enough space for their shoes. Especially when the guy is actually being really nice. My best friend went out with the guy I've been in love with for years and the whole time she was a crazy whining Bitch From Hell. I love her and everything but even I was thinking 'no wonder her dumped you, ungrateful woman. No way I would have treated him like that.
I'm glad you know your way around a lady. You seem to be a really nice guy too so I hope she appreciates you taking her of heart and body x