Help a girl out understanding EDD?

Question

This is my 3rd pregnancy and I still don't understand why they ask for your LMP in order to calculate your pregnancy and how far you are... I KNOW it's estimated however, we clearly don't conceive on our period? 
So why don't they calculate from Ovulation? 
Sorry if this sounds super stupid 😂 It's just ALWAYS confused me! (It's a bit disheartening having a dating scan and being told you're a week behind etc - It just feels like a weeks been add on in your mind) 😂😊

Answer

It’s the first day of your period assuming you conceived/ ovulated on cycle day 14 not the first day of your period. 
If you know when you ovulated, are 2 weeks to that.

Answer

They begin with that date as an estimate because most people will only vary by that by a week or so. But then you have your dating ultrasound and they will calculate to the day, at that point your due date may move.