Imagine you are a student / scientist / dev or someone who generally needs to work with media from around the web. You will download lots of papers/books/scripts/assets, which all need their own place in your local file system, without unnecessary redundances.
The default Downloads folder is fine but after a time littered with setup files, stuff downloaded by mistake etc. And let's face it we are all lazy. Nobody takes their sunday morning for manually cleaning their PCs.
Imagine you could take an image or a pdf file and just drag and drop it to your own dedicated folder for it.
All your electrical engineering work sheets in one folder, all your drawing references in another etc.
Firefox handles all the POST/GET requests in the background and you can go on with your day without losing all the time/energy for totally automatable tasks like this particular file management. I bet you will forget/lose/double-download significantly less files.