Position changes are super important in both obedience and rally! Your dog needs to do position changes both in heel position (without going wide) and in front of you (without creeping in). Platforms can help teach your dog both of those concepts!
The position changes are sit, down, and stand. Your dog should be able to do these in any order — in other words a sit to down, down to sit, stand to down, stand to sit, sit to stand, and down to stand.
You'll need a platform large enough for your dog to stand and lay down on. It doesn't need to be as narrow as a sit platform, since some dogs like to stand with their rear legs a bit wider than the front legs. It can even be something like an agility table, as long as your dog can lay down on it.