When you create one of these folders, you get three types of keys. The Read-Write key allows peers to access and modify its contents, the Read-Only key gives peers the power to access and decrypt the files, while the Encrypted key only allows peers to receive the files -- they won't be able to decrypt them. For instance, you can provide your clients with a Read-Write key to allow them to modify the documents you're working on. But, you can give vendors Read Only keys to show them what you have to offer. If you want to make a backup, say on a cloud service, you can make a folder that uses only an Encrypted key to be extra safe.
Besides the security feature, the latest version of Sync also gives it the ability to run as a service on Windows, as well as to move data to and from an SD card used by Android 5 devices and newer. Finally, if you're on Linux, the update will allow you to only download files you're missing instead of replicating whole folders.