Adjustable Gain?

This may be a dumb question, but what is the purpose of an adjustable gain switch on a preamp?

an adjustable gain switch allows you to compensate for varying voltages of input sources. many such gain adjustments are in phono stages, allowing greater gain for mc cartridges than mm. some companies (e.g., boulder) provide gain adjustments for each input. generally, the lower the gain, the lower the noise floor. -kelly
Generally adjustable gain is designed for the purpose of allowing you to use your volume control at the top of its range which is by far its most linear region. Many volume contols are like Jekyll and Hyde, great in their sweet spot but compressed with poor tracking at the bottom of their travel.