The Party would claim that the purpose for this is to stop the use of words that are unnecessary. In fact, it makes things clearer, less ambiguous, and simpler for the people. Isn't that thoughtful of the Party? The people even believe this about the principles of Newspeak.
However, in truth, what this actually does to a society is dumb them down. When vocabulary has limits placed on it, the ability to have an awareness of the world decreases. Words are the colors to thought and intellect. Much like textures and colors, words can meet quite near the same thing, but not exactly something else.
For example, you can't just become untremendous in your job. That negation of a positive word feels like a fruitless meaning. You could be told you are doing mediocre, or you are slacking. Each of these provide a specificity for why you are no longer tremendous. But this is the Party's true aim, to get people to stop thinking because it is easier to control a people who do not think.