why a high end phono preamp?

I upgraded my phono stage in the past year replacing a 25+ year old unit.  At first, I tried a sub $1000 unit made by a well respected phono preamp designer.  Giving it weeks to burn-in, I could only conclude that my cherished vinyl was being ill served.  So...after absorbing a modest financial hit, I traded in that new phono stage for a significantly more accomplished unit.  Night and frickin'day!  Besides a new life breathed into my vinyl, the shocker was how LP surface noise was minimized by the upgraded unit as it navigated those unmusical noises more adroitly.  In short, at least in my experience, investing in a fine phono amp is well worth it.  

I'm sure someone more technical can explain this better but a phono preamp is taking a very small signal and magnifying it many times and also applying a riaa equalization to the signal. No small feat and how well it is done is definitely going to affect sound quality.

You don't have to spend a fortune for an outboard phono stage, the new Parks Puffin punches well above its weight at $400. My $1000 phone stage is for sale now after getting the Puffin.

+1 jbhiller " A quality phono preamp makes a huge difference"
IME, this is absolutely true. I've used preamps that were stated to have excellent phono sections (melos & Jeff Rowland) & didn't get the sound quality I was looking for until I went with a dedicated phono preamp. The first one was a sonic frontiers signature phono amp. The second is Audio Alchemy's latest with optional power supply. Big difference.

"Is it the Hegel that is responsible for delivering superior sound, or is it the preamp?"   Everything, from the quality of your vinyls, to your speakers/room acoustics, is responsible for superior sound.   A chain is only as strong as it's weakest link.  

With the best amps and speakers, you can't make up for a loss upstream. A good phono section is mandatory for good vinyl playback!