Simple answer, it sounds better. I have a Ayre KX-5 Twenty, it wasn’t cheap. I tried the bypass from my CD player straight to the DAC as I have a volume control on the CD player. It sounded lifeless. There’s a reason the preamp is one of the most expensive components in a system and one of the most important. I can’t get over how much I’d been missing before I got the Ayre preamp.
I’m a true believer if you want a top sounding system, get the best preamp you can afford.