It's going to take more than a single post to answer that question. There are so many reasons that it's hard to keep track of them all.
From an ecological point of view, robots can help replace a factory-like system of crop production that slowly turns fields to deserts with a more nature-compatible and nature-reliant system that can restore fertility.
From a dietary standpoint, the use of robots can vastly increase the variety of crops produced on any piece of land, and help make alternatives to a heavily grain-based diet more affordable.
From the perspective of the consumption of nonrenewable resources, like petroleum, assuming they aren't just doing the same old things, robots can get by on a fraction of the energy spent on tillage, and because they would do their work slowly and continuously instead of all at once, could get much or most of their energy from renewable sources (sun and wind).
There are other reasons, as well as much more to be said about these. I'll get to it in time.