Who knows.
He has different businesses: one is (as I remember the main) brand of artificial eatable meat, so maybe he might want to fight traditional farms. Another one reason could be a promising tech for next gen nuclear plants he's investing in, so may want land for them.
According to him, it's just real estate investment. Not only he's buying a lot of land, but also other billionaires like Bezos, or hedge funds.
But controlling the farmlands means they control key resources on them: food and water. That gives them power, and allow them to control what chemicals they put there, or maybe control or affect the availability or pricing of certain water or food. Keeping farmlands in less hands also means their price grows, so they profit just from that too (real estate speculation).
Regarding water, there are places where there are shortarges for the people (at least here in the South of Europe) but at the same time they are also opening new huge datacenters that use a shit ton of water to cool down those servers. And do you know who has maybe the biggest chunk of dataservers? Microsoft.
And well, farmlands for pasture (food for cows or sheeps) is valued lower than farmlans to grow eatable stuff there like corn, soy etc. So they also profit from buying lands for animals and if possible to turn them into a place to grow vegetables.
For them (or Gates in particular) is a better business if you stop eating cow and move to become a vegetarian/vegan or eat that artificial meat crap.
Video explaining part of it: