I consider a file management app as a must have for the Kindle. With it, you can access the file system on your Kindle Fire. It will give you the freedom to manage your files and content on the go. You will be able to move, copy, and manage files, just like you would on your computer. Without a file management app, you will need to plug your Kindle into your computer to access the file system.
Personally, I use ES File Explorer. It has a look and feel that reminds me of Windows Explorer and it’s free.