It's important to stay hydrated during your workouts. If you neglect to drink enough water, not only do you risk dehydration, you're going to get a lot less out of your workouts as your body begins to rebel against your demands for more physical output.
Keeping a water bottle in your gym bag also allows you to continue hydrating on the drive home, instead of stopping at a convenience store and stocking up on pricey bottled water or sugar-laden soft drinks.
Sure, water fountains can be found in any gym that doesn't pride itself too much on torturing its members. But stopping what you're doing -- be it lifting weights, riding a stationary bike or even swimming (and no, don't drink that water) -- to walk across the gym, wait in line, and then do your best to avoid the germs of those who used the water fountain before you gets old quickly. It's much better to bring your own bottle, fill it up at home or at the gym's tap, and be able to access it as you please while you burn off last night's cheeseburgers.
Now cool down, shower up, and see the next page for lots more information on gym bag essentials.