the big use cases are inter-computer communication and nvme ssds. pcie 4x16 gets you 400 gbps Ethernet. 6x16 will be 1.6 tbps. for SSDs, it's the difference between needing 4 and 1 lanes to saturate your bandwidth. a modern 2u server at this point can have an ungodly amount of incredibly fast storage, and can expose all that data to everyone else without a bandwidth bottleneck.