The Saffire and Scarlett microphone preamps provide between +10dB and +55dB of gain to the input signal before the converters. The gain required to obtain an acceptable signal level will depend on several factors including the input source and the type of microphone.
For example, signals with a high SPL like those from acoustic drums and guitar amplifiers aren't likely to require much amplification so it's normal to set the gain on the lower range. This is particularly true if a sensitive condenser microphone is utilised. Quiet signals such as vocals will require more amplification, particularly if used with a dynamic microphone.
The gain change by the potentiometer isn't linear and it's not unusual to require setting the gain towards the high extreme to get a reasonable signal level.