double pointers

[g3n0m3]

so I have this code for inserting a node in a binary tree what I didn't understand is the use of double pointers, why doesn't work with regular pointers

ptr = tree root
mData = the variable that stores the data
mLeftPtr,mRightPtr = pointers of treenode objects which point to other treenode objects

template<typename NODETYPE>
void Tree<NODETYPE>::insertNodeHelper(TreeNode<NODETYPE>** ptr, const NODETYPE value)
{
	if (*ptr == NULL)
		*ptr = new TreeNode<NODETYPE>(value);
	else
	{
		if (value < (*ptr)->mData)
			insertNodeHelper(&((*ptr)->mLeftPtr), value);
		else
		{
			if (value > (*ptr)->mData)
				insertNodeHelper(&((*ptr)->mRightPtr), value);
		}
	}
}