Low profile cards

As ct said, they are meant to fit into low profile cases. Also, it is often an effort to cut costs since it ends up on a smaller cheaper PCB. For a card more complex than a 9400GT or Radeon 4550 having that GPU on a low profile card means the PCB is less crowded and often that the GPU is less effectively cooled. 9600GT low profile cards have pretty packed PCBs and run a little hotter than normal profile cards that enjoy a bigger heatsink and more spaced out components.