In Part Three, Chapter Three of 1984, when Winston is being tortured in the Ministry of Love, O'Brien explains to Winston why the Party wants power:
"The Party seeks power entirely for its own sake."
This surprises Winston because he expects O'Brien to repeat the Party's official line: that it seeks power for the "good of the majority" because people are "frail" and "cowardly creatures" who cannot rule themselves. This is what the Party tells the people of Oceania and how it justifies its totalitarian style of government.
In contrast, as O'Brien confirms, the Party seeks power purely for the sake of having power. It does not want to improve people's lives, to make the world better or even to make itself rich and famous. It wants nothing but control over other "human beings," to create the "laws of Nature" and to be "masters of the universe." For the Party, power is the key to immortality.