What I use my line driver for:
I have .2 (as in 2 tenths) of a volt out from my HU, as a result of never going higher then 45/60 on volume, and never turning bass past 0dB on HU.
My line driver takes that weak signal and boots it to 5V. Thats what I have it set at.
My RE 12.1 now only needs the gain to be turned almost all the way down to get what I want out of it.
Thus, the amp uses less power, becuase it's not working as hard to amplify the already weak singnal. This way, less heat, and overall less amperage draw, which my amperage available is limited.