The human body is made up of roughly as much as 60 percent of water, therefore it is imperative that we keep replenishing it. This gains even greater importance in cases where there is a lot of fluid loss due to sweating. You will typically stress a lot when you take up outdoor activities such as cycling, running, mountain biking, etc. This is where hydration packs come in really handy.
Mayo Clinic pings the daily requirements at around 3 liters of water for men and just over 2 liters for women. Of course, that is only a rough estimate and it could vary from person to person depending on a whole host of factors. The pertinent takeaway is that hydration is absolutely essential.