Reasons to spay a female dog: Unspayed female dogs have a %25 chance of developing Pyometra after every heat cycle. Pyometra is an infection in the uterus in which the uterus fills with puss and bacteria and in a few short days can rupture and kill the dog, it's a horrific thing.
In addition to risk of pyometria, intact females are about 10% more likely to develop mammary tumors by the time they have had their first heat cycle. After the second heat cycle that risk increases to 25%. Mammary tumors can be very invasive and are often fatal.
Last reason: Go to your local shelter and see how many dogs are already going to be put to sleep because of unwanted puppies from people who didn't get their dogs fixed.