The key scene, where we learn of the Party's true motivation, can be found in the third chapter of book 3. There we discover the true nihilistic calculus that drives the Party's policies of suppression and domination.

Ultimately, the Party has no interest in justifying its actions, nor does it see any need to. For O'Brien, the party seeks to assert domination over people simply because it can, and because it wishes to. This desire to dominate and control: this is itself the motivation driving the Party to act as it does. This is an insight which O'Brien expresses himself, when he tells Winston to "imagine a boot stamping on a human face—forever." That image serves to express the true face of the Party (and more broadly speaking, the nature of totalitarianism itself).

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.

\From a political point of view, the party's desire to seek power is to consolidate control over the lives of its citizens and of Oceania. I think that Winston's statement is transparently valid, one of the few that can be deemed as such. The idea of the power wishing to seek power for its own sake, to greater maximize control over the lives of its citizens and of the nation, in general, is what defines their style of government. This form of rule eliminates the private realm entirely as a check or limitation. There is a great deal of intrusion and interference in the lives of its citizens because power and control know no limitations and few boundaries. It is with this in mind that the party is a construct and being of power, to be able to exert control and force in any realm it deems appropriate. Thus, the limitations on sexual conduct and the restriction of personal thought. The party seeks power because it is fundamentally driven to eradicate any possible threat to its control, and any possible alternative narrative.

In 1984, Orwell has O'Brien tell Winston why the Party seeks power.  He says that it seeks power simply for the sake of power.

O'Brien tells Winston that the...