The documentation goes into this question in a fair amount of detail:
You can add a Listener object to the Controller, which provides an event-based mechanism for responding to important Controller state changes. This is the approach used in this tutorial – but it is not always the best approach.
The Problem with Listeners: Listener objects use independent threads to invoke the code that you implement for each event. Thus, using the listener mechanism can introduce the complexities of threading into your program. It becomes your responsibility to make sure that the code you implement in your Listener subclass accesses other parts of your program in a thread-safe manner. For example, you might not be able to access variables related to GUI controls from anything except the main thread. There can also be additional overhead associated with the creation and cleanup of threads.
Avoiding Listeners: You can avoid using Listener objects by simply polling the Controller object for frames (or other state) when convenient for your program. Many programs already have a event- or animation-loop to drive user input and animation. If so, you can get the tracking data once per loop – which is often as fast as you can use the data anyway.
See the documentation linked regarding the Controller object for information about polling: https://developer.leapmotion.com/documentation/v2/cpp/api/Leap.Controller.html
