a preamp has MANY benefits.
Input source selection.
Volume and balance controls.
Tone controls, YES, tone controls. They are invaluable in my opinion, especially if the source music is not perfect, also room layout, speaker placement, so much can affect the sound and tone controls can go a long way to 'compensate' for any short comings.
I use Baxandall tone controls in my custom builds, one the best tone control circuits out there. I also offer a tone defeat switch, very handy when a 'pure' line preamp is desired for certain listening sessions.