@wonder91,
It is how they can quickly monitor how well you are doing at following their advice and orders.
You can tell them that you are eating six meals a day, but if when they weigh you, you've lost another 5 pounds, they'll know you aren't being very honest with yourself or them.
As to how it helps them with a diagnosis for your migraines, as many before this have said, your lack of nutrition and severe weight loss are most likely the underlying causes of your many other perceived illnesses. Your priority needs to be getting your health back to good levels. Most everything else will resolve, or can be addressed once you are on the road to better health.