The combination of the visual image and the threatening ostinato music on the soundtrack makes it clear that the camera is, in effect, the shark moving almost blindly through the water. Spielberg could have opened his picture with cleverly edited inserts of a real Great White (as Michael Anderson did in Orca the Killer Whale), or he could have teased the viewer with a fin cutting through the water (as Danny Boyle chose to do in The Beach). Instead, he used the point of view shot.
POV shots are the equivalent of first person narratives in fiction, but, unlike novels, films rarely maintain them over an entire story. When they do - as in Robert Montgomery's 1946 version of Raymond Chandler's The Lady in the Lake - they seem more gimmicky than involving. A more considered and more effective use of the technique was in Rouben Mamoulian's erotically-charged interpretation of Dr Jekyll and Mr Hyde (1931), which contains a bravura transformation scene shot from the main character's point of view. Indeed, the POV shot is a staple of horror films such as Halloween and The Shuttered Room where it is used to mask the identity of the killer.
It was partly this tradition that inspired Spielberg to adopt the technique, but mostly it was done out of that mother of invention, necessity. Pre-production storyboards and on set photos suggest that it was the original intention to include shark 'money shots' much earlier in the movie. Spielberg ended up not showing the shark because it refused to perform on cue, and, quite frankly, because it looked a bit rubbish. When we do finally get to see it in close-up - rearing up out of the ocean to take its final victim in its jaws - we're so caught up in the story that we don't notice its rubber teeth bending as they bite down on a screaming Quint.
