it really depends on the install-- if you can get adequate volume without one, and without turning the gains all the way up, than you shouldn't need one. but if you have to keep the gains pegged at max, than it might be a good idea to look into one.
I use a Pioneer DEQ9200 DSP digital processor. It takes one pair of inputs and splits the signal into a 3 way active. I use a Phoenix Gold PLD1 to bump the signal before going to the DSP and then from there to the amps.
Data911 M5 system
RR and iGo8PC
Pioneer head unit
Pioneer DEQ-9200 digital processor
Phoenix gold line drivers
Ultimate and Visonik amplifiers
SMT 3 way active front stage
Digital Design sub woofers
3 runs of 0 gauge wire
I use a line driver. PC motherboard and sound card audio outputs are rarely up to the task of driving long cable runs in a noisy car environment. The output from my USB sound card (M Audio Transit) is weak and from a noise rejection standpoint, you want as hot a signal as possible going back to our amps. I use a preamp EQ with a built in line driver and aux input for my Car PC sound and I love it. It is an old school Precision Power PAR-225. It also has 5 bands of equalization with adjustable center frequencies and bandwidth. A picture is below. I am still working on fabricating a pocket to go between the head unit and the preamp. Audio Control currently makes a similar unit and there are cheaper knock offs that I have no experience with. I bought my PAR-225 used for $75.00. New, these originally sold for about $300-400. There is a later model with similar features but no aux input (PAR-245). These units have line drivers that can deliver up to 9 volts peak to peak and in my system, the sound quality was improved significantly.