The increase in compute density in servers over the past several years has significantly impacted form factors in the enterprise. Whereas you used to have to move to a 4U or 5U chassis if you wanted an 8-core machine, these days you can get there with just a single socket in a 1U or 2U chassis (or smaller if you go the blade route). The transition from 3.5" to 2.5" hard drives helped maintain IO performance as server chassis shrunk, but even then there's a limit to how many drives you can fit into a single enclosure. In network architectures that don't use a beefy SAN or still demand high-speed local storage, PCI Express SSDs are very attractive. As SSDs just need lots of PCB real estate, a 2.5" enclosure can be quite limiting. A PCIe card on the other hand can accommodate a good number of controllers, DRAM and NAND devices. Furthermore, unlike a single 2.5" SAS/SATA SSD, PCIe offers enough bandwidth headroom to scale performance with capacity. Instead of just adding more NAND to reach higher capacities you can add more controllers with the NAND, effectively increasing performance as you add capacity.