You can also use the desktop app only. You could also choose to automatically fill forms, like Lastpass does, through the browser extension. For this to work you need OR Dropbox or Owncloud or WebDAV. In my situation I wanted to use the browser extension, but did not want to move out of my local environment. That is why I set up a local server to connect to.
I am not claiming that Buttercup is better then Lastpass or Keepass, etc. It is what it is.