Because their articles about how they handle laundry on the international space station include lines like this:
"To construct my planter, a spherical core is needed. An old pair of underwear worked well. We have supplies on Station sufficient to change our underwear perhaps once every 3 to 4 days, so I figured there might be a few nutrients in there as well."
That's right: Microgravity dirty-underwear tomatoes. For science!
I have a fistful of shiny rocks for the first good writer to use that in a story.