This app is perfect for those who use particular websites which requires to be running on the browser for long periods. As this kind of websites could be closed accidentally you have to open a new browser tab/window every time. Singlebox solves this problem by gathering user-desired websites into a single window. It is like Flow (https://fluidapp.com/) on steroids and is much more useful than Flow.
I use Singlebox for Reddit, Whatsapp, Twitter and some other forum websites.
The developer, Quang Lam, is very responsive to support mails. I definitely recommend Singlebox.