Also there's different degrees of it. Some women are not affected by it at all, some are only very slightly affected, then you get some who are more severely affected by it. Also the effects themselves differ. Not all women who are affected by it get moody or angry. Some just get more sensitive and maybe weepy. Some are affected physically (outside of usual expected cramps), with nausea, vomiting, headaches and cravings being quite common on top of the PMS, which can exacerbate it.
I suspect another contributing factor is blood loss, especially for heavy bleeders as it can lead to anaemia if not properly supplemented with nutrients which can make you feel pretty crappy and moody amongst other symptoms.
My usual symptoms include general fatigue and feeling a bit emotionally sensitive (which I only really notice if I'm already unhappy about something). The physical effects are far stronger than the emotional effects. I don't tell people when I'm on my period and act completely normal, because the effects are weak, under control and don't need to be expressed.
I could understand some women wanting to warn someone they work/hang out with if they get bad enough periods. I've had a couple of bad ones in my high school years and was floored, it was like being proper ill and in agonizing pain, and you really do need some emotional as well as physical support. I don't think I could maintain composure in that state every month.