My Dad went to a zendo regularly for quite some time, when I was much younger. As a child, the concept of sitting and only sitting was mysterious to me, so I asked him why he did it. He told me that he wasn't sure, so he asked the head teacher at the zendo, and later recounted the exchange to me, which I have always valued as something of a modern koan in its own right:

Dad: "So, sitting will definitely produce positive results if you do it for long enough, right? What if I sat for 20 years?"

Master: "You mean, a guaranteed amount of enlightenment, however small?"

Dad: "Exactly."

Master: "The only thing I can guarantee you will get after 20 years of zazen is hemorrhoids."