According to Apple my early 2011 MacBook Pro only supports up to 8GB. I've been happily running it with 16GB for years now without issue. That said according to Intel's i5-4690 and i7-4790K specs the maximum supported memory is 32GB. More details in the datasheet: "Using 4Gb DRAM device technologies, the largest system memory capacity possible is 32 GB, assuming Dual Channel Mode with four x8 dual ranked DIMM memory configuration". See also an Intelligent Memory FAQ entry on the issue. While I'd be happy to see someone get their hands on some 16GB SO-DIMMs and prove otherwise, my limited guess is that 64GB support in the Late 2014 iMac is unlikely. As for why anyone would need or want 64GB RAM in a desktop, any sort of analytics over about-that-size datasets is the first thing that comes to mind. Perhaps also developing and testing in-memory databases, or cluster technologies with bunches of VMs. The 1TB PCIe SSD is quick, but RAM is still a lot faster. Smoke 'em if you got 'em.