Value of a preamp?

It can't add anything but can color the sound in a manner that some find positive.  

Jerry

Can greatly enhance the overall sonic characteristics of a system. Tonality, dynamics, soundstage, and so much more, when it is the right fit with everything else.

It can offer so many benefits if you have the right one. Soundstage, clarity, dynamic contrast, slam & dynamics, imaging, details, separation of instruments & vocals, etc. 

A good tubed preamp will greatly enhance your sound period.

There are always lots of a variables, but that was certainly the case when I upgraded. I’m sure technology has progressed since then, but the move from a Hafler DH100 to a Lazarus Cascade Basic tube pre was pretty significant. It brought the system to the next level, and gave me the ability to roll tubes to tweak it. It drives my Dyna/VTA amps extremely well, which is one of the critical parts of that elusive synergy. It became a pretty incredible foundation to build a system around. There is no single right answer, but even a modest tube pre sure worked out well for me.

I agree with most above. And yes, there are very good solid state preamps.

Most important component in the chain...

A good tubed preamp will greatly enhance your sound period.
 

I disagree, as there are satisfying SS preamps also such as Dartzeel, Audionet, Griffon Commander…  it depends on the execution and of course user preference 

A good tubed preamp will greatly enhance your sound period.

Vintage Preamps are loaded with Features that Integrated Amps typically omit:

Balance; Two MM Phono Inputs; Tape Loop; Mode Switch; Filters including Fletcher Munson 'Loudness' for Low Volume Listening; Match Gain for 3 inputs (volume stays the same as you change Inputs)

more info here

The one value that rarely gets mentioned is the ability to control the interconnect such that it eliminates any artifacts. However, this will only happen with a pre-amp that supports the balanced line standard (AES-48) and connected to an amplifier that supports the standard as well. It's how I do it in my system.

Fair number of preamp lauding threads on here. A few more people praising their dacs volume control sans external pre. "Richness" was the term used that I thought described it best, in my system anyway.

+1 @soix  and @jl35 

+1 soix 

It can greatly improve the overall performance of a system.