This is a unity gain amplifier, a.k.a. a voltage follower. In theory it doesn’t alter the audio information of the signal at all. What a buffer does do is present a high-impedance load to the input signal, and outputs a low-impedance signal.
In practice? This prevents the signal from high impedance source (such as guitar pickups) from being adversely affected by, say, long cable-runs, or by being plugged into a low-impedance devices (such as, in my case, a volume pedal).