There is no need for wolves (dogs' wild ancesters) to be socialized outside of their packs because they will more than likely not run into strange wolves and if they do, they oftentimes do what they need to in order to get rid of the intruder or threat. However, since dogs live within human society and will encounter other people and dogs, socialization is crucial so that they DON'T behave like a wild wolf and attempt to kill, maim, or chase away the threat. If they do that, they can ultimately end up having to be euthanized for aggression.

That is why dog parks can be very beneficial for socializing.