WiFi is how tablets get onto the internet and download content. The LeapPad Platinum comes with WiFi built-in.
The tablet needs to connect using WiFi during its initial setup. This is to register the device with LeapFrog, and to download any necessary included apps. Until the tablet is setup online, it will not be possible to use it properly.
Once these, and any future, apps are downloaded from the LeapFrog App Center, they can then be used offline, although WiFi access may still be needed for some features, such as peer-to-peer gaming.
WiFi allows the Platinum to connect to the internet, and for a child to view any of the online content that is approved for children by LeapFrog, using LeapSearch.