That depends on the food. My parents mostly grew fruit, and berries especially are very expensive at the supermarket.
The other things they grew were also equal or better to the most expensive produce at the most luxury supermarket, like tomatoes that tasted of tomato rather than the watery, red golf balls they sell in Asda.
There are other benefits of fiber, such as transmitting non-Ethernet signals such as USB, DisplayPort, HDMI, etc. You can run these things over Ethernet, but they are really shoddy and unreliable.
It allows you to centralize more things. For example, you could just have a monitor, keyboard, mouse, and a hub in a room for a computer, where the computer is actually somewhere else (such as a server room).