General purpose of an EQ- To flaten out frequency responce to achieve a more accurate musical reproduction. To help 'balance' peaks, and dips inherent in both speaker systems, as well as the nasty peaks and dips caused by the car audio enviroment.
General Purpose of a Line D.- To increase the voltage of the RCA signal to create a stronger signal, so that gains do not need to be set as high, allows a lower gain setting, for less clipping, and overall a bit better sq. There is also a marginal SPL gain when using one typically, keep in mind I did say marginal.