Disney first started using the biometric turnstiles about 10 years back to stop people from sharing season passes, and then a few years later they added it for plain old multi-day tickets.
It used to be that there were folks lingering in every Denny's and IHOP parking lot on International Drive in Orlando, buying and selling partially used multi-day theme park tickets. The marginal price increase for adding an additional day drops quite a bit for every day you add to a ticket - so you could have bought a 10 day ticket for not all that much more than a 5-day, used half, and sold the rest to someone else.
The fingerprint scan is done so that the ticket is biometrically locked to the person who first uses them, thus killing that secondary market for partially used tickets.