If you start searching, I think it defaults to scope "This Mac". That's probably right for most cases. If you want to open a Word doc named Fnord, you'd kind of hope Finder would... find it... wherever it was. But you can also click next to "This Mac" to switch it the context of the directory you're in.

Also, cmd-shift-G (the Finder shortcut for "Go to Folder...") will let you start typing a path.