[fooqux]

Because I don't have a double-yellow line painted between my property and my neighbors? Machine learning is fine and all, but you need something for the machine to learn from. Without some kind of outside influence, my grass looks the same as the grass across the invisible property line.

Not only that, but you need a system to account for wheel slippage, etc. No matter how accurate your compass, how awesome your internal map, you're eventually going to go off course. Best case, your bot mows your neighbor's property. Worst case, it wanders into the busy street, or into your pond. Systems like these beacons or underground wire help ensure this doesn't happen.

Source: I have a half-built robot lawnmower on my workbench.