We've seen a bunch of custom SSD form factors with the arrival of Ultrabooks as well as systems like the MacBook Air and Retina MacBook Pro. The need is simple: standard 7mm or 9.5mm SSDs are too big for some of these machines, and you don't actually need all of that volume to build a fast drive. mSATA cards work as a small form factor solution but they are only available in a single size. We need flexibility both on length (to allow for higher capacities) as well as on the interface side to enable higher performance. The NGFF specification gives us just that.