This applies to the Scarlett and Saffire Ranges
The Saffire and Scarlett microphone preamps provide between +10dB and +55dB of gain to the input signal before the analogue to digital converters.
The gain required to reach an acceptable signal level will depend on several things including the input source and the type of microphone you're using.
For example, signals with a high SPL (Sound Pressure Level) like those from acoustic drums and guitar amplifiers aren't likely to need much amplification so it's normal to set the gain on the lower range.
This is particularly true if a sensitive condenser microphone is used. Quiet signals such as vocals will require more amplification, particularly if used with a dynamic microphone.
The gain change by the gain knob isn't linear and it's not unusual to set the gain towards the high end of the dial's rotation to get a reasonable signal level.