Cowork flips the usual chat around. Instead of uploading a file and copying the reply back out, you connect a folder and Claude works on the files where they live: reading them, editing in place, and building real deliverables (spreadsheets with working formulas, decks, documents). You describe an outcome, and it does the multi-step job.
It only touches folders you connect, and deleting anything always needs your say-so. Match the oversight to the stakes, keep backups, and keep sensitive files out of the working folder.