Answer
The purpose of the copper coil is to decrease the wave speed, which in turn decreases the frequency of the musical tone that is produced.
Work Step by Step
We can write an expression for the speed of a wave along a string:
$v = \sqrt{\frac{T}{m/L}}$, where $m$ is the mass of the string
The copper coil increases the mass of the string which decreases the speed of the wave along the string. If the wave speed decreases, then the frequency decreases.
The purpose of the copper coil is to decrease the wave speed, which in turn decreases the frequency of the musical tone that is produced.
