out End in the model ensure this result.
These two constraints are not needed when start=l and end=5 because the nature of the data guarantees the desired result.
However, for start=3, node 3 allows both input and output flow (arcs 4-3 and 3-4) and, hence, constraint inStart is needed (try the model without inStart!).
The objective function maximizes the sum of the output flow at node start.
Equivalently, we can choose to maximize the sum of the input flow at node end.
The model can find the maximum flow between any two designated start and end nodes in the network