A few stray thoughts....hope they help :-)
The music is menacing and the 'dun dun' beat is relentless, like the shark
The music resembles a large beating heart
The shark moves silently and invisibly through the water, the music establishes, for the audience, a 'signature sound' for the shark for the rest of the movie. They hear the music and know the shark is present even though they may not see it. This heightens dramatic tension.
Possibly compare the music to other films which use, for example, the villain's approaching footsteps, heard but not seen, for dramatic effect