Not everyone has fast connection to download games. Microsoft wanted to give customers a choice to get retail games and install it on hard drive so after that disk won't be necessary. Valve games have retail releases b/c of the same reason.
Because they fully understand going 100% digital is not a viable business solution, and in doing so they would have to compete with Steam, who treat digital games as digital games and charge accordingly. By offering both however, they can trick people to pay retail prices for digital files.
"Microsoft demands simultaneous multiplatform releases" - Type that into your search bar and you'll see why you should buy Rayman on Wii U, not ports.