When going very long distances on a bike, it is easy to become dehydrated. The wind blowing over your skin sucks the moisture out of your body. Wearing a full riding suit helps, but often is not enough. If you don't want to get off the bike every hour or so, then you better have water on your bike that you can drink while riding. Drinking water in sips is one of the best ways to keep hydrated. Your body seems better able to absorb the water without causing you to need to piss as frequently. There are many ways of doing this, and I have used a few of them. On my Harley, I used two 1 liter water bottles hanging in the fairing lowers and ran a small hose to each. On other bikes I just had a water bottle that I could access while riding.
On my ST1300 I put Platypus bottles in the tank bag. Platypus is similar to a more familiar container called a Camelback. I like the materials that a Platypus is made out of, and the size and shape. I have one 3 liter bottle, one 2 liter, and a 1 liter bottle. Filling up all 3 gives me a gallon and a half of water. That is seldom necessary, only really needed on a multi-day desert ride. Having this much capacity allows for easier water acquisition though. I prefer NOT to buy water. I generally get it from the hotel rooms or water fountains. Sometimes though, water has to be bought. Small bottles of water are expensive, but gallons are cheap. I often buy a gallon of water for less than the pint bottles cost. Having the extra capacity makes this easy. When you don't need much water, you don't fill them all up, saving a bit of weight.
The bags go in the tank bag. The hoses come out the zipper and are attached to the tank bag using a device designed for ID badges that zips and out. This allows the rider to drink the water, then let go and let it snap back into position. The ends of the hoses have what is called a bite valve. This keeps the water from pouring out the hose until the rider bites the valve, opening it up.
Some people complain about getting a shot of hot water from the hose when first sucking. This can be prevented by blowing into the hose both after taking a sip and before. This makes for less water in the hose to heat up, and pushes any that made it there back into the container.