Do I need an iMac Pro? Absolutely not.
Do I want an iMac Pro? Absolutely yes, and I have much use for it as well.
Alas, as I've just earlier this year purchased a brand-new 2017 edition of the 5K iMac, upgrading again so soon is blasphemy against sound financial judgement. The 5K iMac is plenty fast and serving me quite well. It chews through photographs and 4K videos with great ease.
The iMac Pro can do it exponentially faster though! Imagine what I can do with 10 CPU cores, 64 gigabytes of memory, and a workstation-class AMD GPU. One of the best investments is to throw money at saving time, and products like the iMac Pro is entirely about shaving precious seconds off a video render or photo import. It's worth it if your livelihood depends on it.
Unfortunately for me, I have a proper day job and the digital production stuff is but a hobby on the side. Ergo while I can use the iMac Pro, it doesn't make sense for me because my paycheck doesn't depend on it. For sure this might change in the future, but for now, I'll be more than alright with the 5K iMac.
Besides, there's the modular Mac Pro and pro-level monitor still to come from Apple next year. Even if I were inclined to plop over $5K down for the iMac Pro, I'd at least wait until those other products have seen the light of day. Options are fun like that.
In the meantime I'll live vicariously through the Youtube creatives that have bought or will buy the iMac Pro.