This. I thought I had allergies that stuffed up my nose and sinuses my entire life. Then I started tracking my water intake, drinking the proper amount, and boom, my entire head cleared up. I'm so mad at myself for not realizing this decades ago.
For me it was better digestion and less headaches. The downside is kidneys aren’t ready for that amount of water (or are, depending on perspective) and you need to pee much more often, like every 1.5-2 hours. I use around 3l/100kg/day.
Turns out, I had spent most of my life dehydrated. I feel so much better when I drink a ton of water.