There are various reasons to add a line driver. I highly recommend one but only if you have a need. A line driver is usually a built in benefit of an EQ. Kicker, JL Audio and Audio Control are just a few of the dedicated line drivers. And some line drivers have the benefit of built in zone controls.
If you have a triple preout high voltage source unit and you are not dividing your RCA outputs then you probably have little use for a line driver.
If you have a low voltage source unit, inordinately long RCA runs or you are splitting your RCA outputs among too many amplifier inputs, then a line driver will either recover lost voltage or give the needed voltage.
You can always turn up your amplifier input gains to offset low voltage and obtain higher volume but there is inherent compression in a low voltage source unit and you're also raising the noise floor.
I don't necessarily equate a line driver to higher volumes as much as the added dynamic range. Good recordings/pressings can have 18dB or more dynamic range. That is a 1 to 64 power ratio between the softest and loudest (peak) music material. So a line driver can restore so to speak lost contrast which I hear as added clarity. Then its up to your amplification and speakers to carry on the available dynamic range. But at least the signal is dynamic from the source and that determines the limits of everything that follows in the audio path.