Perhaps the single most important concept to grasp in sound engineering is the concept of gain structure. Your gain structure will determine the overall signal-to-noise ratio and distortion level of your sound system.
If the channel gain is too low, you are likely to not get optimum performance. You will have to push your fader volume so loud to get any volume out of the system that your noise floor will be tremendously high.
If the channel gain is too high, you will not have the ability to get your input and output channels at useful operating points/within unity range. This will effectively leave you without the ability to get a good mix. It will also cause you to find feedback quicker than you can say, “Oops.”