Tomato plants tend to lean and droop as the tomatoes mature and get heavier. Planting on fences can provide support and help keep squirrels or other creatures from getting your tomatoes before you do. The fencing creates a protective structure around the plant that gives you a sturdy place to secure branches as the plant grows and starts to bear fruit. It should be tall enough to protect the entire plant at full height.
If you found this helpful. I have been planting tomato on my fences for the last 4 years after I saw what happen when I planted them in the middle of the garden on cage was not pretty and was not convenient for me an my kids to maintain because we cant see the inside of the cage when it is fully grown. Our kids love the way it easy to pick and see what growing on fence!