The party seeks power for the sole reason of obtaining power. A government who's people live in fear of it is in total control and can create and remove laws without the countering opinions of the general public. The book is a futuristic view of what a 1950's communist / fascist country would become in over 30 years. So to clearly answer your question, the party seeks power because the obtaining of power creates an illusion to the population that the government is an immovable, unstoppable, unquestionable entity born and nurtured from the fear of it's own people.
