A reasonable question. I used to work in the visual imaging media industry for close to 3 decades and I used to work with the movie, music and imaging people before I got outsourced to someone who's cheaper and younger and more mobile (casual/freelance). Anyhow, the Macs have always been associated with the creative arts when they had a viable desktop graphical system before Windows got into the scene with their Windows. Today, both have comparable graphical interface, but the stigma of Macs being better for creative arts stuck on, so Macs are almost always relatively more expensive than a comparable PC. A typical creative artist, like my sister, focus on buying the right tool at any reasonable cost as long as it has the specs to complete the tasks at hand. Most artists I talked and worked with in the past (I worked in the media industry for close to 30 years) prefer the OSX graphical user interface more so with Windows. I own both Macs and a PC and I prefer Macs for the creative arts side and use the PC for more mundane business tasks managing our finances and taxes.
A Mac Pro is designed to be used and bought by professional in-house media companies; at least most of them in the past due to its vast expandability and by a few individuals who can afford its hefty price tag and simply wants the top of the line fastest Mac money can buy. Nowadays, it's more contractor/freelance based, so less of the media companies are buying them new than before and most of them had transitioned to PC based media systems as they are providing more bang for the buck so to speak. The video editing software like Vegas Pro for PC and Adobe Premiere Pro (works on both MacOS and Windows) and the imaging software like DXO Photo Lab 2, Adobe Creative Suite, Photoshop etc are available on both platforms so it no longer makes financial sense to buy a Mac Pro any longer. And Logic Pro X for the music industry by Apple has also its own PC based equivalent. So there's now no longer any exclusivity of software once held only by the Macs for creative artists. You can be as creative with a PC as with a Mac.
You also have the iMac Pro and the latest Mac Mini 2018 which provide the performance envelope once held exclusively by the older Mac Pros currently available for sale or even used. There is no definite reason to buy a Mac Pro in 2019, other than you need lots of expandability as most other Macs can satisfy the demands of most Mac users for the majority of the requirements be it in gaming, video editing, image editing and audio editing. And because of this, the cost of the Mac Pro can be quite high initially as it is a speciality computer.
Having said that..
A Mac Pro is made to be durable and is an industrial workhorse as it has workstation components, so it can cost more. It's made to last longer in continuous use like 24/7 if need be compared to the usual consumer and prosumer macs. If you are making money with a Mac and you need a reliable Mac, a Mac Pro cold give you that piece of mind. But then again, today's computer equipment are quite robust and reliable compared to the past so even this notion of the Mac Pro being more durable than other Macs may no longer hold true depending upon your usage. My sister's Mac Mini 2,1 is still going strong after more than a decade of use and my MacBook 2007, now my dad's personal computer, still works well. The only quirks is they both use an older OSX which does not allow browsing in government secured websites for security reasons.
Today; computers are more modular..
A Mac Pro made in the past are based on a tower concept where your expansion peripherals are inside the tower. Most modern PCs are this way, but I am seeing that people really want more modular expandability rather than buying a full big tower. The current Mac Pros are designed this way and the newer Mac Mini 2018 are also designed this way. TB3 (Thunderbolt 3) has the bandwidth to drive a lot of amazing peripherals which makes 4K video editing and the future 8K video editing possible, so expandability via TB3 makes the Mac Pro less compelling need to be used as a sole workstation platform for most Mac users.
4K and 8K video editing and AI based photography applications..
I think this will be deciding factor between someone who might need a Mac Pro versus a new Mac Mini 2018 is in the support of applications that make heavy use of the GPU and that once you factor in the cost of the GPU with a lots of ram and a bigger SSD drive on the Mac Mini 2018, it probably make more sense to buy the Mac Pro instead.
Smooth 4K video editing is very processor intensive and make use of the GPU with its own onboard ram and AI based photography applications are also processor intensive and make use of the GPU with lots of ram. In regards to AI based photography applications, it can be a difference between processing an image in 30 mins with an Intel GPU with 1.5-2Gb of ram as opposed to less than 1 min with a powerful Nvidia/AMD GPU w/4Gb to 8Gb of ram. The newer Mac Pro 2019 or even the 2013 model can manage and cope with this. The new Mac Mini 2018 would probably need an eGPU attached to a TB3 port to provide additional GPU support, but not all apps support external eGPU. And if it does, this raises the cost of the Mac Mini 2018 closer to a stock Mac Pro 2013 or even a 2019. Would it then be more prudent just to buy a Mac Pro rather than buying a Mini 2018 instead?
If you're not planning to edit 4K footage commercially, then a Mac Mini 2018 would probably suffice without adding a eGPU. But if you are doing this commercially with 4K exclusive, then a Mac Pro would probably be a better choice.
