If you have a 4k TV and disposable income.  But your right....  

Your question is essentially "Why do people want games to look better."

If you have a 4k TV and want your games to look better then buy one, else don't.

No. I'd rather upgrade my PC for that money.

Someone who hasn't experienced sharper images and more fluent gameplay won't understand it of course.

Well people rebought the slim versions of last gen consoles too, despite being the exact same thing.

But this time youre also getting better looking games along more console features. What people forget is that the PS4 Pro is a hell lot quiter than the OG PS4, it offers you to record footage in 1080p over 720p too.
For little more than a PC graphics card youre getting also a big boost in performance and graphics, just try out games like Nier Automata or Tekken 7 on PS4 Pro.

The answer is very different depending on the company's marketing.

Sony: "You shouldn't. We are still committed to the PS4, the Pro is just an addition. If you're interested, here's Mark
             Cerny talking softly about checker-boarding for an hour"
MS: "It's the next-gen step in gaming! The definitive XBox console and the right way to play!
          WITNESS THE MOST POWERFUL CONSOLE EVER!!!!! $499!!!"

A guess as to which has resulted in lower sales for its base hardware

There's literally an entire industry that's powered in large part by people buying stuff so they can have "slightly better graphics", so there's little doubt that there are people that consider it a worthwhile investment. It's just a matter of how well that sentiment extends to console players, and those undecided between consoles and PC.

This might be the greatest post I've seen in years.

I honestly have no idea why...

People like to spend money I guess

For me the upgrade from PS4 to PS4 Pro was easy.

1. Offers a higher resolution and more stable PSVR expereince, which IMO is the most important and exciting thing going on in all of electronics, let alone games.

2. Takes advanatage of the 4K HDR display, and High res Audio equipment I bought.

3. Gives fast loading times, higher resolution, smoother frame rates, and additional image quality to nearly all itles, even non patched games via Boost Mode.

It may not be enough for everyone, but it is right in line with what I want. Like I have said before, we have $200 TVs and we have $50K projectors, we have $100 sound bars, and $10K Reciever + Speaker set-ups. Everywhere else you go in electronics you have the option of unnecessary, overpriced, cutting edge hardware, I have wanted this in the console business since I was playing PS1. I am extreamly happy with Pro and X, and hope it leads to even higher end model. I would love a $1,000+ console to hook up to my A/V center that offers industry leading processing.