This new optional add-on lets you sync your public cloud folders with your NAS (Dropbox, Google Drive, Baidu but no Microsoft OneDrive - formerly Skydrive) in a few clicks (I use Google Drive - that was easy). Will also let you fetch from the cloud if you delete locally or replicate the delete. Really like it.
Other new things include the Quick Connect feature which intends to let you access your stuff from anywhere, Apple file performance has been enhanced and much more. If you have a Synology NAS this upgrade seems worth it.