Story related and can not be missed.
The Grid is the story of Tron: Legacy. Sora and Riku's stories are happening at the same time. Sora's story stops about half way through the movie. Riku's story goes right up to the end of the movie.
The game does not touch on this at all, but I have a complaint. The Tron in the Grid should have no idea who Sora is. In Kingdom Hearts 2 the Tron world you visit is said to be a copy of the original Tron world. In The Grid they explain that this is the world that was copied for KH2. Which means the Tron Sora meets in KH2 is a copy of the Tron in KH3D, ergo, the Tron in KH3D should have no idea who Sora is because it is not a continuation of the KH2 Tron world, even though Tron: Legacy that this world is based on is a sequal to the original Tron.