Lots of articles on this are full of "4D chess" speculations about climate change blah blah, but the real reason for this seems to be very simple: "just to collect rent."
Most of us imagine farmers tilling the soil that has been in their families for generations. But many farmers lease at least some of the land they cultivate. According to Bruce Sherrick, a professor of agricultural economics at the University of Illinois at Urbana-Champaign, about 60 percent of row-crop farmland in the Midwest is leased. The landowners can include investors like Gates.
"Farmland has had a remarkably consistent ability to hedge against inflation," Sherrick says. And it tends to be "negatively correlated" against other investments, he adds: If the stock market is going down, the return on farmland is likely to be going up.