Accessing a NAS at decent speeds. Even for a single 7.2k rpm drive that's rated at 120 MB/s, a gigabit link speed is a bottleneck (120 MB/s = 960 Mbit, add protocol overhead and you're over 1 Gigabit). If you have any kind of array (zfs, raid ...) of drives you need more speed again. If you have an SSD cache (an option even in Consumer NAS now) not having a multi-Gbit/s link is essentially a waste.
Of course you can send 10GbE over copper, but the needed amplification is a major heat source. So if you're running cables anyway you might as well run fiber and be prepared for 100GbE.
Of course you can send 10GbE over copper, but the needed amplification is a major heat source. So if you're running cables anyway you might as well run fiber and be prepared for 100GbE.