Stereo preamplifiers are used to control the sound from a music player to either a power amplifier or a set of active speakers. The essential functions of the preamplifier consist of a number of analog and possibly digital inputs via a DA converter (DAC), input selector, volume control and often also tone control/equlizer.
A large number of Hi-fi stereo preamplifiers also have built-in streaming functionality, which means that you often do not need any other sound sources than the amplifier/music streamer itself.