Moving or copying files from one location to a folder nested deep inside another folder can be troublesome. Apple has a concept called Spring-Loaded Folders. Drag a file on top of a Folder, don't let go of the mouse button, after a pause, the folder opens up, revealing its content. You can keep doing this until you find the desired destination folder. Let go of the mouse button, your file is transfered to the destination folder, and the Finder closes all those folders for you automatically.
If you don't want to wait for the folder to open as you hover on top of it, hold down the space bar forces the folder to open immediately.
Pretty handy UI feature.
If you don't like the concept of spring-loaded folders, you can turn it off via Finder preferences.