Whether you're training in hot or cold weather, you need to drink frequently.
During long-term activity, exercise tolerance will decrease because of the water lost through sweating - unless you adequately replace the lost fluids. For marathon runners, fluid loss can be several as high as 5.4 liters per hour. Also at high altitudes, drinking is important because of the dry air.
If you don't drink enough, you will become dehydrated and your body's total blood volume will drop. Because the heart has access to less blood, it has to pump faster to circulate the same amount of blood - and your heart rate will rise. If you don't replace the lost fluids by drinking, your heart rate will increase, and your ability to perform will decrease rapidly.
So drinking fluids during exercise has several benefits:
Offsets body temperature increase
Minimizes cardiovascular stress
Note that in addition to drinking too little, drinking too much during performance can also be dangerous. Overhydration can lead to low blood sodium levels (hyponatremia). Hyponatremia is most commonly associated with prolonged exertion during high-intensity endurance activities like marathons.