It's wrong to say they were useless like OP suggests. They were very useful. It was a crop you could reliably grow and come back to harvest.

It also stored very well. The breeding was only to make it more useful. It was always useful.

Much of the breeding was just selection. The crops you would pick and store would be larger. So we it came to plant your were using the biggest largest variety every year. A few generations of this would produce notable results. Then even finer and more deliberate selection would be done.