The electrical output from a turntable's cartrigde is very low, and nowhere near the line level output from other components that an amplifier expects. A seperate pre-amp is used to boost the signal from a turntable up to line level.
The job of the phono pre-amp is to correctly match the characteristics the stylus for best signal transfer. A good pre-amp will allow adjustment for the cartridge type (MM or MC) and also cartrige impedance and capacitive loading. The phono pre-amp also applies RIAA equalization curves to the turntable's output to compensate for the skewed frequency profile of recorded vinyl. Some pre-amps also offer additional features, such as USB output for connection to a computer in order to archive records, or sonically clean up old or damaged vinyls.