The browswer adds another layer of indirection you don't need. Why would you prefer to download/use that functionality through the browser rather than straight to your machine?
Sandboxed execution environment. Alway up to date for both sides doing the sharing. And because I don't want to download some unrelated software just to be able to download a file from one of my other devices. And unless I'm building this app for each platform I use, I have no guarantees that the binary I'm downloading is from the code on the github.
