Bonnie the staff puppy, submitted by Charlotte and Alice
It is absolutely vital that you train your dog from as young an age as possible, preferably as soon as you receive it. It is far easier to train a puppy then to train an ageing dog, and so by training the dog as soon as it starts living with you, you will find it far easier to control and care for.
Dogs need obedience training because it will improve happiness for the dog and will of course improve your own happiness, as it is far less stressful trying to command an obedient dog then a wild one. If a dog goes without training and develops behavioural problems, it will be far harder to fully appreciate your dog. For example, it may be unapproachable to other people, and you could constantly have to monitor your dog when out in public. Obviously this isn't going to be very pleasant on you, the owner, or even on the dog itself. The dog would miss the attention from other people, and perhaps even other dogs (if it has to be kept apart from them or muzzled to prevent attacks).

By training your dog not only will it strengthen the bond between the dog and its owner, but it will serve as a potent reminder that you are the one in charge, that you are the "leader of the pack". Training builds a sense of understanding between you and your dog, as well as improving communication and as mentioned, a sense of respect. A dog will be far more likely to respect an owner that has trained it properly; otherwise it will simply try and take control of you, sensing a lack of leadership on your part.
Training your dog can have other important benefits. In an emergency, a dog can easily be controlled by using the simple sit command. For example if you leave the gate to your garden open and the dog wanders out towards the road, if spotted by you or a member of public hopefully they can command the dog to sit, and it will do so. Then the dog can be taken to the owner, without fear of the dog running off into the road or causing an accident. If a dog receives no training then it will be far harder to control, and in potentially dangerous situations this could prove fatal. It will benefit everyone in the long run to have a reasonably-well trained dog. This doesn't mean the dog needs to be able to jump through hoops as and when required, but to just be able to perform the mandatory simple commands - such as being able to sit, or heel.