First, I have to disagree with Marakanetz about the output/input impedance ratios - a preamp output impedance to power amp input impedance ratio of 1:20 or higher will be sufficient. (For instance, if your preamp has a rather high output impedance maximum of around 1,000 ohms, try not to pair it with a power amp having an input impedance below about 20,000 ohms. As a practical matter, most preamps have an output impedance lower - and most power amps an input impedance higher - than in this example, so impedance mis-match between preamp and power amp is not usually a problem anyway). Also Marakanetz, what are you talking about with your references to preamps having to "restore" the "drastically compressed and feedbacked (sic)" output of CDP's? This makes no sense to me at all.
Randalle, the basic answer to your question is gain: active preamps offer additional gain, which might be needed to listen at the desired loud volume, depending on the source material, the source component's output level, the gain level offered by the power amp, the speaker sensitivity, and the size of the listening room.
Also, controlling volume with a passive device can sometimes become problematic due to impedance-matching issues between the resistive element used for volume attenuation and the impedance characteristics of source outputs and amp inputs, as well as those of longer cable runs. Active input and output buffering around the attenuation device, as in an active preamp, can eliminate these potential issues, which can make system matching easier by providing volume control that is relatively insensitive to surrounding conditions, and by relieving the source component from having to drive long cable runs.
Ideally, a preamp could provide gain only when it is required, but most active preamps have a fixed permanent gain factor, which is then attenuated as needed by the volume control - often (as you note) to levels below that of the source's own output level (AKA below 'unity' gain). Generally speaking, the less gain - and the less attenuation - applied to the source's signal, the more transparent the sound, so passive attenuators can have a theoretical transparency advantage for sources with sufficient output drive, as long as none of the above-mentioned impedance-matching difficulties arise. (Click on my Threads and go to the one about preamp bypass testing for more related to your question.)
Randalle, the basic answer to your question is gain: active preamps offer additional gain, which might be needed to listen at the desired loud volume, depending on the source material, the source component's output level, the gain level offered by the power amp, the speaker sensitivity, and the size of the listening room.
Also, controlling volume with a passive device can sometimes become problematic due to impedance-matching issues between the resistive element used for volume attenuation and the impedance characteristics of source outputs and amp inputs, as well as those of longer cable runs. Active input and output buffering around the attenuation device, as in an active preamp, can eliminate these potential issues, which can make system matching easier by providing volume control that is relatively insensitive to surrounding conditions, and by relieving the source component from having to drive long cable runs.
Ideally, a preamp could provide gain only when it is required, but most active preamps have a fixed permanent gain factor, which is then attenuated as needed by the volume control - often (as you note) to levels below that of the source's own output level (AKA below 'unity' gain). Generally speaking, the less gain - and the less attenuation - applied to the source's signal, the more transparent the sound, so passive attenuators can have a theoretical transparency advantage for sources with sufficient output drive, as long as none of the above-mentioned impedance-matching difficulties arise. (Click on my Threads and go to the one about preamp bypass testing for more related to your question.)