One of the main things is that Paperless applies OCR to each uploaded document, so you're not just searching for the message content and metadata, but also the whole content of the document. Maybe some email clients will do this too. It also has a tagging system which can automatically add tags based on document content. Mostly I just like having everything that's relevant in one place, and only what what is relevant. So if I'm looking for concert tickets I don't find the "order confirmed" and other related emails that do not actually contain the tickets.