This DVD came with my boxed set of season 7. It's short but fills in a lot of the events that are often referenced on Game of Thrones. It doesn't go too in depth, but given that there are planned prequel spin-offs, it's understandable that they don't want to tell the whole story. It explains the main points of the events that led up to the political situation of Game of Thrones (Robert's Rebellion for example).
The animation is much more stylised than we might be used to, but for me, that is part of its charm. It is more like looking at a storybook than watching Disney style traditional animation.