Because that 1'', I can put in my hiking purse. The most important is the 40mm never get me down like the 50mm(AF hunting). The front element of pancake of 40mm is so small and no need extra take care and the only lens I feel no need a filter for protection.
If you had both of the lens, you will notice the significant different in built quality.
In my case, I can put the lens in my pocket and not even notice it. The pancake is that compact.
It's not just the size that make this lens a winner, though. It's AF performance is a huge improvement over the 50 f/1.8, and the lens offers excellent image quality... even wide open.