What is the need for the return tree statement in the else statement ?

class Node:
    def __init__(self,mark,left,right):
        self.mark = mark
        self.left = left
        self.right = right

def leaf(m):
    return Node(m,None,None)



def insert(tree:Node,item)->Node:
    if tree is None:
        return leaf(item)
    elif tree.mark > item:
        return Node(tree.mark,insert(tree.left,item),tree.right)
    elif tree.mark < item:
        return Node(tree.mark,tree.left,insert(tree.right,item))
    else:            ## i mean this line and the following line
        return tree     ##