it's an outboard or pedal preamp, which is basically the preamp you find on active basses, except in a pedal. the one you find in your bass is called an onboard preamp.
and what a preamp does is give you the tone shaping options you would normally find on an amp. note that a combo amp consists of a preamp, a power amp, and a speaker. preamp shapes the tone, power amp blasts it to XXX watts, and the speaker is self explanatory. each of these things contribute to the tone. when you use different amps, you will get different kinds of tone. at least if you use your own preamp, the variance won't be that much from amp to amp. plus, you would be more familiar with tweaking your own preamp than having to mess around with a different amp every time.
the additional benefit most outboard preamps have is the DI function. when your bass is connected directly to the house, there isn't even an amp, your tone is at the mercy of the soundman. that's where the preamp gives you control.