One of the key ideas in 1984 is that who controls language controls thought (this relates to the whole business about controling past to contol the future). Language is a complicated issue, but it's basically the only way that we can "think" (some artist would argue with this, but in my experience it is generally true --- try to sit down for a half hour without a word going through your mind.) Some words have a clear one.on.one meaning ... most of us know what "pen" means. Many of the really important words are not that simple; there is no one.to.one correspondence with words such as "love," "freedom," "faith", etc. We do our best with the tools we have, but we can never agree on the exact meaning of these.
In 1984, the Inner Party realizes that if they can control the number and/or quality of words in the language (each new edition of the Dictionary gets smaller and smaller), then the people will not have the words/ thoughts that will allow them to think about/against the government. Take away the language, you may eventually be able to take away the thought of revolt. Control then becomes a very simple issue.
Maybe this is the reason English teachers are so insistant on wide reading and vocabulary development :)
Doublethinks main purpose is to confuse the populace so that they don't know or understand what the Party is doing. The contradictions: War is Peace, Freedom is Slavery, Igorance is Strength, are not all that contradictory. By waging continual war with the other two powers, there is no civil unrest, therefore, there is peace. The people are not free at all, though they are told that they are. They are enslaved to the government and they are told their work is meant to maintain their freedom. By keeping the people ignorant, the government remains strong. Doublethink is, according to 1984, being able to maintain two separate and contradictory thoughts simultaneously. By creating these newspeak words and ideas, the government controls people. Another idea that the government maintains is that whoever controls the past, controls the future. At first glance, this may seem contradictory, but it is very true in the story. The Party is constantly rewriting the past to keep it in accordance with the present and the party's plan for the future. By changing the past, the Party is never proven wrong.