Its purpose is to suppress thought by severely curtailing both the conceptual vocabulary and permissible grammatical structures of Oldspeak. As Orwell describes in the Appendix: "....a thought diverging from the principles of Ingsoc should be literally unthinkable, at least insofar as thought is dependent on words." 
Douglas Kellner writes:
- In 1984, "Newspeak" is the new totalitarian language which replaces "Old English." The aim of Newspeak is to narrow the range of thought so that an individual could not even think critical or subversive thoughts. Potentially critical terms like "freedom" are formally defined into their conceptual opposites ("freedom is slavery"), or are simply eliminated from the dictionary and everyday language. In this manner, critical language would wither away as the number of words which allow differentiation and critique was increasingly reduced. 
It is a terrifying notion, not least for its plausibility and the games we see played with words and meanings by those in power today.
- p. 312, 1984 - George Orwell, Penguin, 1984