Where you actually want a pre-amplifier, naturally, meaning sources with lower output. Another option when using it as a pre-amplifier for a power amplifier but too much gain without turning the volume down will overload the output stage.
Example setup:
Device 1: 3V @ 0dBFS.
Smartphone: 0.5 – 1V @ 0dBFS.
A gain of 9dB makes such a device similarly loud than a normal line out.
