If I'm not a gamer, what are the reasons I might need a higher-end GPU? I'm buying a 27" iMac and having trouble understanding why I may need a higher-end GPU over the next 4-5 years (intended usage).  I do lots of picture editing (Photoshop, Lightroom), movie playing (1080p on an external monitor), and some small time video editing with iMovie.  Is it superfluous to upgrade to the higher end gpu for my purposes?
For your current needs, the higher end GPU will speed things up a bit, but won't be absolutely essential.  However, upgrading your GPU now will make your iMac a bit more future-proof, allowing it to continue to run applications like these at a decent performance level in the future, as they will need more power as time progresses.