Value of a preamp?

I've only heard the Aries Cerat in passing in an unfamiliar system (which didn't impress), but not in a comparative sense.  Every system I've heard with an Audionet preamp (and amps) is always great. And admittedly they were carefully curated systems.

Besides the items you mentioned--impedance matching and volume control--receiving multiple inputs and switching among the inputs is another important function of preamps. 

But, I would consider the other side of the coin. If you're not using analog volume control, you'd be using a digital volume control. And, digital volume control is anything but simple. Here are some of my takeaways from this forum post with the Roon programers on digital volume control:

1. changing the volume digitally requires the hardware to re-load the song on the fly
2. Roon leverages the streaming transport or DAC to handle digital volume control, even though Roon's upsampling happens at the server level [the Roon Core]
   1. As a result, the quality of the digital volume control you experience will vary by the quality of your downstream devices
3. changing the volume of a DSD track, digitally, forces the DSD file to be resampled to PCM 
   1. So, unless your streamer or DAC upsamples to DSD, your DAC will now receive a PCM file

You may find other takeaways from that thread. Also, that thread is a few years back, so some of the info may not be current.

I tried upsampling with Roon, and didn't experience a difference at the time. I then tried upsampling with HQPlayer's dozens of dithers and filters. That was interesting, but I never felt I like got a result better than casting a normal signal to the streamer. 

Currently, I just allow my pre-amp to handle volume control. But I may have a change of mind with different gear. (I currently use a Peachtree Nova 300 for DAC and preamp duties.)

Just from what I've observed, the guys who have good results with digital volume control use a streamer or DAC that upsamples (and often to DSD) and also seem to use an FPGA DAC. In broad strokes, here are the major DAC camps as I see them. (1) Those guys with FPGA DACs seems to prefer upsampling everything to DSD. (2) Those guys with R2R DACs seem to prefer bit-perfect signals, especially 16-bit PCM files. (3) Those guys with chip DACs can be a mixed bag. My Peachtree Nova 300 definitely prefers to see 24-bit streams. Also, DSD doesn't sound as good as PCM with my unit and to my ears, for what that's worth. 

So, if your goal is to cut out an analog preamp, or to have the convenience of adjusting volume digitally, you might consider a streamer that upsamples well, like a Lumin, and/or some kind of FPGA DAC, like from PS Audio or Chord. I have some KEF LS50 Wireless speakers. They sound good, and I believe their DSP engine handles volume control. 

I’ve gone back-and-forth with this since moving to high-sensitivity speakers. I've determined that an active preamp is essential.  Current gain is key even if you don't need extra voltage gain.

Invaluable?

The VALUE is in the features a preamp provides. There are MANY different types and features. To me, the 2 main features are Input Switching and Gain Control.

Now of course if you have only 1 source and it has volume control, then by all means try it without a preamp. It may sound perfect. But it also might sound even better with a good preamp, or maybe not. It depends.

But most people need features. Like me, they need input switching, balance control, tone controls, headphone output, etc. It’s not a conspiracy. A preamp is a tool that does a job, and in some cases, you don’t need one.