Because they show each note and how to play
The conductor is in charge
The interval is called the octave when two different tones blend so well when sounded together that they practically seem to combine into one tone. Octaves are notes that is heard as being the same in spite of being higher or lower in actual pitch. Octaves from the science perspective are pitch intervals connected by two factors. It is a central mystery of human cognition in hearing pitches linked by powers of two as presence the same note. The capability to notice octave equivalency is perhaps constructed into the brains and it is not restricted to humans. The rhesus monkeys have been exposed to be capable to detect octave as some other mammals.
