This is as it says, its the pre amplifier some time called the control amp now. Its job is to match the input impedance of the input device, and the amplification stages.

Then tone equalise and frequency deviation, required in the sound recording procedure, this is normal with record pick ups on record decks. The next stage is in the tone controls, this is normal to make it sound better in the environment you are in and to combat the defects in the speaker systems to room area.