As noted in iMac 5K (Late 2015) with 64GB: the Key to Professional Usage, the late 2015 iMac 5K changes the game by accepting up to 64GB memory, which makes it viable for professional photographers, videographers, etc*.
But it’s more than just 64GB memory, as discussed here in two articles. One covers the advantages and limitations of the late 2015 iMac 5K vs 2013 Mac Pro, the other discusses how to build a high performance workstation based on the iMac 5K.
For my work here at diglloyd.com, the iMac 5K display (5120 X 2880) is a time-saving (and enjoyable) way to proof the high-res output images for my photographic publications, which are up to 24 megapixels: a 5K display is 14.2 megapixels, whereas 4K UltraHD is only 8.2 megapixels. The sheer viewing pleasure of 5K is one reason to get the iMac 5K, if only thinking of it as a 5K display with a free computer included.