well, there are a few things:

1. using the app to take photos (in a scan sort of mode, where it trims it to be at right angles), really quick and easy, no matter where i am.
2. remote access - i can view all of my documents where ever i am.
3. easy & sophisticated search. I have my documents assigned to people (me, wife, child, etc). I also assigned them to things like payslips, tax, shares, legal documents, education docs, receipts, etc. it also helps to automatically tag them to some degree of accuracy
4. Automatic dating, it is quite good at picking out the date of the document, as seperate to the upload date. and it is easily updatable if it is wrong
5. OCR - the documents content is searchable!
6. Ease of tax time. I have some financial year views that make it really easy for me to do my tax (Australia), and i dont need to go hunting for paper that has faded in the heat and is no longer legible.
7. folders - the documents are placed in a folder structure of your choosing. if you change the details in the document meta-data, it will move it to the correct place.

so, whilst a folder structure would work. this is SOOO much easier, and provides much more functionality as it is not just storage. it also has WAF!