The initial purpose of keeping the information is completion. I sheepishly admit to digital hoarding, and this may be feeding that desire. To me, it's easier to scan a document and tag it, rather than importing its information.
I need to keep things like receipts for large purchases for insurance, expense, and warranty purposes, bills and account statements, tax documents, and even things like the rare paper letter I get (e.g. my former tax preparer died last year. If I were to be audited, I'd need some evidence that she's dead. I have a letter from her next of kin and coworkers saying that she died.)
I need original paper for SOME receipts, things with raised seals such as birth certificates or car titles, and other unique items that the originality of the paper would increase its authenticity in a court of law.
What you do seems very similar to what I want to do, perhaps with the exception that I'm a metadata nut and want to be able to search things a little easier, should the need ever arise.