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A preamp is one of those essential items for your studio. If you use a condenser microphone, which requires 48v phantom power to work, you'll need a preamp to provide it! The audio signal from microphones is weak, so they need a preamp to translate it into a stronger "Line level" signal.
Correspondingly, what does a preamp do for a microphone?
A microphone preamp is simply some sort of device that amplifies (makes louder) the microphone signal. Microphones generally have a very low, unusable output. Plugging them into a microphone preamp allows you to bring that level up to a usable (line) level. Any device that has a microphone input has a mic preamp.
Thereof, do I need a mic preamp if I have a mixer?
The preamp would plug into the TRS (Line in) input on your mixer, that way it would not go through the built in preamp. Preamps, even expensive ones are unfortunately not magic devices that make everything sound awesome, so no, at this point you do not need one.
Conclusion. The sound contribution of preamps is not so much in its frequency response but in the texture it imparts on the sound. However, a preamp shapes the sound to a much lesser degree than one would think. Usually, its sound character only becomes obvious at high gain settings or when you drive it into distortion