No matter what you think of Uplay (PC gamers know it as a future digital distribution platform and as facilitator of horrible always-on DRM masquerading as something else - particularly for PC gamers), what Ubisoft is trying to do with it on Wii U is interesting because it adds a number of features to its games on the platform. Polygon has compiled a pretty extensive list of what the new Wii U Uplay app adds to Ubisoft's games including extra game modes, achievements, and rewards.
All of these things are unlocked with what Ubisoft calls Uplay points, which you can apparently earn by completing different challenges in a given title.
A list of those actions - along with the corresponding reward or unlock can be found at Polygon. Games supporting Uplay's extras on Wii U includes Assassin's Creed 3, Avengers Battle for Earth, Just Dance 4, Rabbids Land, Your Shape Fitness Evolved 2013, and ZombiU.