There are many obvious reason to spay and neuter your pets. One is - and this is hard to realize but important to know - there just aren't enough homes in the US for the number of dogs we have. So if you choose to let your dog get pregnant - and even if it's an accident, you chose it by not spaying and neutering - for each of the pups you have in your litter that you find homes for, you have killed a pup in the shelter. People HATE it when I say that because they know it's true. It's just how the numbers, unfortunately, fall. Know that and imagine it if you choose to breed.
IMO, every dog (and cat) that isn't proven show quality (ie in the show ring, in the performance ring, out in the field, etc.) should be spayed and neutered. Period. It takes YEARS of research to become a good breeder. For instance, how many of you know that if you breed a merle-colored dog with another merle-colored dog that you will get blind and deaf puppies that will either need to be killed or have very difficult lives? All because you didn't know one very, very simple and "duh" fact about breeding. Breeding and controlling genes is filled with such pitfalls. Does your dog carry a mutant gene that is sleeping? How do you know? By knowing the health history of EVERY ancestor of your dog back several generations (8 is best).
From an agility/canine sports perspective though, neutering too early can cause real issues structure wise for any dog potentially headed for an active canine sport. Studies show that dogs neutered before the bones have quit growing have different bones growing at a different rate. Therefore, a dog neutered early will grow slightly out of wack due to the changes in hormones. I, personally, do not neuter my dogs before they are 12 to 18 months old because my dogs are agility prospects. However, I also am EXTREMELY careful that these intact boys NEVER (and I mean never) escape their yard to go visit a female in heat. When my dogs are done growing, they are neutered. Even though my dogs are exceedingly good agility dogs (one is an agility champion five times over and the other has qualified to try out this year to represent the US at the Agility Worlds) I do not believe in breeding them and killing other dogs in the shelter.
Really quickly as I'm running out of room, spaying and neutering also decreases dramatically the incidence of many types of cancer - including mammary and testicular. It can save your dog's life.