The main role of Winston is a platform for Orwell to describe and show his vision of this dystopian future that he has created. Through Winston's eyes, we can learn both about the workings of party members, their everyday lives and routines, the philosophies of the party itself, and even the life of the proles that exist outside of the minute control of the party. Orwell needed a character that was a part of the party itself, but also someone who could show the life of the proles; for that, he needed a discontented party member who would wander about and show the lives of other people. But, being a party member allows Orwell to explain, in detail, the inner workings of the party itself, and its many manipulation techniques. Winston's discontentment and rebellion also is staged in order for Orwell to show just how horrific and awful the party can be, and how far it is willing to go to brainwash its citizens when they rebel. Winston is the perfect character to show how the party gains complete and total control, both through subtle means, and blatant tortuous means.
Through Winston's point of view, we see how the party works, but also how it is awful. His misery relates the misery of being a human being in this society. The themes of control, brainwashing, and totalitarian government are shown through is discontent. Through Winston we are allowed to meet Julia, a fellow discontented citizen, and because of Winston and Julia's rebellion, we are able to meet O'Brien, party devotee and enforcer. Winston's life in his apartment building allow us to meet other types of characters--tattling children brainwashed by the party, the mindless, sheep-like followers of the party, and the haggard, lifeless women and citizens that survive day to day under the party's rule. He is a character that encounters all other types, allowing Orwell to show just what living is like in their society, no matter what type of person you are.
Winston is a very carefully created character, one with all of the right traits to show a complete and rounded viewpoint of the dystopian society that Orwell paints. I hope that helped; good luck!