A regular chat is like a sticky note: handy, then lost in the pile. A Project in ChatGPT works more like a labeled folder on your desk, where the brief, the reference files and your notes on how you want things done all live together, and the chats you open in that folder start with that shared background.
OpenAI describes Projects as dedicated spaces where you gather context, files and instructions. You find them in the left-hand menu: create a project, give it a name, then add context by uploading files and setting instructions for that project. OpenAI calls projects containers that preserve evolving context around a workstream, so a new chat inside the project starts from that background instead of a blank page.
Projects earn their place on work that runs for weeks and comes back again and again: a client account, a monthly report, an event you are planning. A one-off question, like rewording a single email, is faster in a plain chat.
The honest limit: files you upload are copies of the version you had at the time. If the budget changes in your spreadsheet, the uploaded copy does not update itself. Keep the source document as the version you trust, re-upload it when it changes, and check figures against it.
Use a Project for work that keeps coming back: put the source files and one or two instructions in once, and every new chat in that project starts with them. Treat your original documents as the record, and re-upload them when they change.