I think the most interesting new features are the ability to add in as many dimensions as you want and the ability to edit the seed of these dimensions individually (you could have 20 unique and pre-determined versions of the overworld if you wanted). It could also be used to get a specific terrain generation for one dimension without sacrificing the benefits of the other dimensions' terrains.
To download the other ones you need to make a folder in the versions folder for minecraft and put the client and JSON file for the versions in there. They all need to be named the same aside from file extensions. Once you do that, you will be able to choose that version when making a new profile with the minecraft launcher.