Renderers may cache the data they need to render the figure. When properties of a figure already in the collection are changed, (e.g. a key point of a curve is moved or new points are added to a path), then Invalidate must be called manually to inform the renderer that cached data may be invalid.

When a figure is part of another figure (composite or transformed figures), then Invalidate needs to be called on the root figure because the render data is usually cached in the root figure.

Reference