It is widely stated that concentration of cholesterol, triglycerides, lipoprotiens in blood plasma varies widely during the day because of eating, digesting, etc. As far as I know, fasting for 12 hours before blood is drawn for these analysis has been SOP for past 50-100 years. Purpose, of course, is to give reproducable results.
The drinking of water before having blood drawn facilitates that process. Makes it far easier to draw blood because blood is more fluid from the additional water. Decades ago I was involved in medical situation where both fasting and no drinking of any fluids was required for 12 hours before drawing blood -- often problem with blood being too thick to get adequate sized sample -- so, we would have to drink a big glass of water, wait 15 minutes and nurse tried again, usually successful.