OpenCV, findContours - Why do we need a

This is my question. When I have a variable where the contour points are stored:

<vector<vector<Point>> contours;

There are points/coordinates stored in:

contours[0][0].x; contours[0][0].y;
contours[0][1].x; contours[0][1].y;
...
contours[0][n].x; contours[0][n].y;

But no points/coordinates are stored in:

contours[1][0].x; contours[1][0].y;
contours[1][1].x; contours[1][1].y;
...
contours[1][n].x; contours[1][n].y;

or

contours[n][0].x; contours[n][0].y;
contours[n][1].x; contours[n][1].y;
...
contours[n][n].x; contours[n][n].y;

So why do we need a vector of vector of points, instead of a vector of points? A vector of points would be sufficient or not? But openCV wants a vector of vector of points.