Why do we need the first if statement to remove a node from a binary search tree?

Below is a method to remove a node from a binary search tree. I don't get why we need the first if statement. If the binaryTree is null, why do we return binaryTree? Shouldn't we return null? I will really appreciate your response.

public BST remove(int valueToRemove) {//12
    removeHelper(this,valueToRemove);
    return this;
}

public BST removeHelper(BST binaryTree,int valueToRemove) {
    if(binaryTree==null){
        return binaryTree;
    }
    //larger->go to the right
    if(valueToRemove>binaryTree.value){
        binaryTree.right=removeHelper(binaryTree.right,valueToRemove);//to remove
        //smaller->go to the left
    }else if(valueToRemove< binaryTree.value){
        binaryTree.left=removeHelper(binaryTree.left,valueToRemove);//to remove
    }else{
        if(binaryTree.left==null&&binaryTree.right==null){
            return null;
        }else if(binaryTree.left!=null&&binaryTree.right==null){
            binaryTree.value= binaryTree.left.value;
            binaryTree.left=binaryTree.left.left;
            binaryTree.right=binaryTree.left.right;
        }
        else if(binaryTree.left==null&&binaryTree.right!=null){
            binaryTree.value= binaryTree.right.value;
            binaryTree.left=binaryTree.right.left;
            binaryTree.right=binaryTree.right.right;
        }else{//when you find the value to remove
            binaryTree.value=getSmallestValue(binaryTree.right);
            binaryTree.right=removeHelper(binaryTree.right,binaryTree.value);//to remove
            return binaryTree;
        }
    }
    return this;
}

public int getSmallestValue(BST binaryTree) {
    return binaryTree.left==null? binaryTree.value:getSmallestValue(binaryTree.left);
}