its one of the things the long range readers can do. Basically think of anything where Disney would want to know who you are without you having to tap the RFID reader. Disney can actually use this in multiple ways, some to provide unique personalized experiences(whatever that may be) or the use some people are more worried about, which is tracking your movements within the park, to either profile you personally for more CRM opportunities, or for metadata type analysis on things like traffic patterns etc.
And by tracking I mean gathering your ID as you walk past readers in the park. The RFID chips do not provide an actual way to track you unless you walk by the reader.And even then all they get is that your magic band walked by a certain spot at a certain time.