With a stand-alone application once you download it in your file system you know exactly where it is and how to create backups etc.

A "browser cache" is just an opaque bit of storage. What if you need to update/reinstall your browser or want to switch? I wouldn't trust important data to it.

I generally feel uncomfortable how so many applications are browser-only these days. The thought of having important data in a tab that you might close by mistake at any moment is uncomfortable. Browsers should really only be used for fleeting content, not productive work.