The gain control on a mixing console or hardware device is a small signal amplifier.
To set a proper mix gain structure the overall S/N (signal to noise) ratio should be maximised. The signal input is the first stage of the signal path of the recording procedure, and it is important when using the gain control of the recording device to capture the best signals to record.
Setting a signal gain too low will mean the engineer has to compensate after the recording (use a compressor) for low-level signals by increasing output volumes; this results in increased noise floor levels. Too much gain on the signal will overload the input circuitry and result in a distorted signal.
Every microphone, every instrument and each vocalist produces different quality source levels. The gain control allows the engineer to bring each signal source to an equal level for recording purposes.