Okay, so the goal is to go from a wiggling metal string to a bunch of 1's and 0's in the computer. A few things are needed to do this. First the wigglement of the string is converted to an electrical signal - pickups. Then we need to change from the impedence of the pickups(thousands of ohms) to something closer to what professional line amps are used to dealing with(microphones tend to be hundreds of ohms) - impedance transformer, typically a DI box. Then we need to amplify this signal to a level that the analog to digital converters will be happy with - preamp. Then we need convert the amplified (line level) signal to a stream of 1's and 0's - digital to analog converter. Then we need to transfer the 1's and 0's from the memory in the converters to somewhere useful in the computer - digital audio interface.
So our chain looks like this:
string(mechanical signal) -> pickup(analog electrical signal) -> DI(still analog) -> micamp(still analog) -> converters(digital audio data) -> interface(digital) -> computer.
This is assuming we are recording our instrument direct, micing an amp you get:
string -> pickup -> preamp -> amplifier -> speaker -> microphone -> micamp -> converter -> interface -> computer.
Products exist which bundle several of these functions in to one unit. At the lower end of the cost spectrum it is more common to see the digital interface and analog to digital converters on one unit. It is also common to see DI, micamp, converters and interface on one unit. This is probably what you have there. With units such as this the pre is often not so good, so even if you do not need a preamp, it could be worth using a line out from a decent bass amp anyway.
The balanced line out you see on some bass amps is doing the job of DI and mic pre.
