The role of the preamplifier is to adapt the source signal before it is amplified by the power stages. As the first stage of amplification, it is the key to a good signal-to-noise ratio. Indeed, the more a signal is weak in intensity, the more it is sensitive to interference. It is this point in particular that will make the difference between a good and a bad preamp.
That said, the goal of separating the main power stages and preamplification becomes explicit: to limit the interference between the high level and the low level.
In addition to that, some preamps will have a built-in DAC, a balance management, a frequency filter or a CD player.