Or does the harp need to detect at which point the beam is interrupted?
Any stringed instrument makes different sounds depending on where the string is plucked. Plucking near the end causes less vibration (i.e. sound) than plucking near the middle.
Since the idea is to replicate the strings with laser beams and the vibrations with computer-generated sound data, it is necessary to know where along the string the "string" was "plucked".