A branch fallen to the ground is either already dead or is in the process of dying. Soon that branch will be perfect kindling for a fire. If the branch remains on the vine, then the sap of the vine keeps that branch alive and productive. Rather than being perfect firewood, the branch is the perfect receptacle for the fruit of the vine.
Jesus compares Himself to the vine and calls us the branches. Without Jesus, we are dead. Without Jesus, we bear no fruit.
“As the branch cannot bear fruit by itself, unless it abides in the vine, neither can you, unless you abide in Me,” says Jesus.
“By this My Father is glorified, that you bear much fruit and so prove to be My disciples.”
Naturally a branch on a vine will produce the fruit of that vine; naturally Jesus’ disciples will produce the fruit of the Spirit, which “is love, joy, peace, patience, kindness, faithfulness, gentleness, self-control.” (Galatians 5: 22 – 23)