Pratical use, no, pixel peeking while getting your nose close to the monitor , sure, some might see a difference.
It's mostly bragging rights , though to push those extra pixels you need a better GPU, which is the main issue with the 5K iMac .
Or you've never done 4k video editing. Its very nice with a little window resizing to display the viewer in 100% size while still having your timeline, source clips, and effects windows available as 5k gives you 50% more screen real estate than 4k. The only other solutions are mulitple displays (one of which must be 4k res) which takes away time in shifting focus or clicking in and out the full screen viewer.
Also, my camera does 28 MP images so we are nowhere near maxing out resolution for different needs.