This unofficial client is based on the web service provided at Messenger.com, which allows you to access all the features without any restrictions. With it you can chat with your contacts and groups; send pictures, videos, and stickers; and make phone or video calls. Besides all that, it also supports desktop notifications, so even if the application is minimized, you'll see the incoming chat notifications.
Obviously to use it you need a Facebook account. That lets you sync all your activity with the rest of the clients on other platforms, be it the chat feature on the website or the different Facebook smartphone apps.