Everything. I have databases for home and work. In the home database I have top-level folders for me and my wife (for personal stuff we each might store), documents, invoices and receipts. The Documents folder has all sorts of delineation - banking, animals (pet passports, vaccination history, etc), businesses, insurance, investing, legal, medical, peoperty, radio (N1GMA), taxes, vehicles, vendors, etc. I scan everything. Contracts, receipts for things I might want to return, receipts for tax deductions or expenses, manuals (usually a PDF download), bank statements, property documents, spreadsheets related to home projects, literally everything that exists on a piece of paper that might be valuable in the future. I'm from the US, live in Chile, and have residency in Chile and Panama. There are a lot of legal docs related to those processes that go into Devon. I own a business in Panama, so everything related to that is also stored in Devon. I travel, so if I'm in the US and need a document related to something from another country, I know that it's on my laptop or accessible from my phone via the sync.
I also use Devon to rip web pages, either to save as a bookmark or as a one-page PDF, so when I find something useful for work or around any idea that interests me, I'll put it in Devon instead of a Chrome/Firefox/Safari bookmark. I never remember what's in the URL of a bookmark to search for it, but I do remember the content. I can search in Devon, find the PDF, and if necessary, click the link to go back to the webpage.
I write content for part of my job, so I'll use Devon for brainstorming or outlines, all in Markdown. I've written a little snippet for Keyboard Maestro so the Cmd-N in Devon asks me what type of document I want to create (markdown, plain text, or rich text), and then it creates that in the folder. If I'm doing basic stuff, like taking notes for a meeting, I just work in raw markdown, but if I'm actually formatting something like a blog post, I'll click the "Open Externally" button to open it in Macdown. I'll make action items and after the meeting transfer them to Taskwarrior for execution. I don't need to remember what happened in every meeting - I just need to remember to read the notes.
I'll collect images for blog posts and store the whole thing in a subfolder under Projects/Content/Blog/{article-slug} so that everything is nicely organized. I use the Search function for those "I know I saved something about {foo} a few weeks ago" moments. Otherwise, I jump right to the project I'm working on and am confident that everything related to it is within that folder.
I've had too many times where I've gone looking for something that I saw on my desk a few months back, and it's gone. Invariably I'll have the receipts for every other thing _except_ the really expensive thing that just broke. With Devon I no longer have that problem. I'm pretty disciplined about scanning receipts every weekend, but I definitely have to stay on top of it for the system to really be effective.