"...one was not obliged to act a part, but it was impossible to avoid joining in...the rage was an abstract...emotion."
The hate serves to provide an outlet for people's aggression so they don't focus on the real problems in Oceania. It also provides a constant stream of scapegoats for the party to blame people's troubles on. It distracts the population and keeps them hating what the party wants them to hate.
This slogan is propaganda attempting to make the citizens of Oceania believe that they have everything they want. The citizens are taught to believe being free means being a slave. Meaning, if they were free they would be unhappy. They are taught to believe that war brings peace, and that being oblivious to the things going on around you is strength. All three of these sayings are meant to convince the citizens that life could not possibly be better with another form of government. This prevents citizens from rebelling against the government