Are Preamplifier’s Relevant Today or just a Hinderance with Digital Playback ?

There are some very high quality integrated amplifiers, both old and new, and most would be quite happy with the results.  Often outclassing some separates.  However, the best performing systems still use a separate preamp and power amplifier.  

I suppose it could depend on how many sources. I have no use for a preamp. 

You just need to match what is called  consumer line level which is nominal 310mv RMS to your power amp sensitivity. Whilst CD players boast 2v figures this does not occur, as all the media you can find will be 310mv RMS, so your power amp needs to be close to this too.
https://en.wikipedia.org/wiki/Line_level

The long and the short you can then use passive attenuation which is represented by resistance, vs adding via active preamps which can't help themselves but add various flavors of reactance, in so doing distancing you from what is possible.

The benefit is you will hear what your source component can actually do,

Totally system dependent.  I don't recommend digital volume contrl. It will cut fidelity in most cases.  DAC output vs volts needed for full output from your amp help you decide.  Speaker sensitivity also comes into play.  Some systems need gain from preamps some don't.  Active preamps will change the sound.  Its up to you whether it is better or worse.  

One way or another, a system needs volume control and source-switching (assuming it has more than one source). If you have only one source, and it's digital, you could get away with software volume control only. In my limited experience with it, software volume control seems to compromise SQ compared to a good hardware implementation.  

At any rate, in between a computer that runs DSP (like HQ Player) and a power amp, there generally needs to be a box with USB/other input (at least) and loudspeaker outputs. How is it a hindrance if that box supports source switching, volume control, IR remote, etc.?  Unless you just want to connect powered speakers (or headphones) directly to the computer.  But then you need a keyboard, display, mouse/trackpad to control the computer (instead of connecting a "headless" computer to a component audio system).  Seems like more of a hindrance than a preamp.