Pseudepigraphy is like cheating on a test in reverse: Instead of passing off someone else's work as your own, you're claiming a famous person wrote your work.
When an author is famous, their work may carry better authority and a better reputation. Pseudepigraphy happens when someone intentionally attributes a work to a famous historical figure to ensure that it's taken seriously. This was common with ancient texts; for example, a scribe might claim that Aristotle authored a treatise that he never actually touched.