How does control of language translate into control of society by the Party?
We think in words/language. Although we are never certain that the words we use "point" at the same reality as does the same word when used by another person (eg. people often use the word "love" and often mean different things by it --- this is true of most abstract nouns). If we can remove a word from the language, then we may be able to make it impossible for the users to express the concept, to think, ... and the concept itself may fade away. Take the word "freedom" for example. We may have trouble agreeing on what this means, but it's clear that if those in power could eliminate from speech/thought, then they would be in a better position to control us. Concepts such as "oldlife" are created to describe our life before the new order, and when they are given a negative connotation, they make it less likely that we will feel attachment to our private life. Eliminate "oldlife" and you are one step closer to the society that controls the past and the future.
Language must be guarded, and we must hold speakers feet to the fire, as it were, when it is abused, misused, or just shrouded in ambiguity. For example, the word "rich" is thrown around in our world (tax the rich) a great deal, but do we really know what "rich" is? Does it have a fixed meaning? Or does it mean whatever the powers that be want it to mean.
We treasure our language because it is at the root of our freedom; that's why the Newspeak dictionary is so important --- and why, contrary to our own, it is always shrinking.