One of the many reasons 2001: A Space Odyssey is so memorable is that the music used throughout is powerful. It is used to express different moods at different times. There is grandeur and a feeling of expansiveness to "Thus Spake Zarathustra", used when intelligence is used by the primates and shown leading to majestic scenes of space flight. Music used during the long trip to Jupiter is slower and more low-key, showing the sometimes boring aspects of long-term space voyages. Kubrick used music by an experimental composer, Gyorgy Ligeti, in scenes of strangeness and uncertainty such as when the primates begin investigating the monolith, and in the bedroom at the end.
An interesting point is that there was a complete score written for the movie, but Kubrick had been using the music eventually used as he worked on the movie. He decided to stay with what he had while he worked.