Hey Mike.
Yes...having a dedicated GPU can definitely come in handy with some computing tasks. The difficult part with GPU's (or a computers graphics capabilities)...is getting benchmarks can be difficult. There are sites out there that can compare the performance of an integrated or dedicated GPU to other integrated or dedicated GPU...to give you a relative performance comparison. What you don't know is how they will perform inside of a specific computer configuration (this info is harder to find).
Thus if you compared the graphics abilities of a 2013 27" iMac dedicated GPU to a 2018 Mac-Mini's integrated GPU...you never know...because of the 5 years in-between...the Mac-Mini could be faster.
But for example...even if the 2018 Mac-Mini had 2x faster graphics. If it takes your 2013 iMac 48 seconds to import some photos...and with the 2018 Mac-Mini it took 24 seconds (2x faster)...that still may not be enough to warrant the purchase of the Mac-Mini. And hey...with the 2013 iMac...you got that super awesome 27" display!!!
- Nick