Stripping away the original metadata (like your location or camera type) for privacy. The Anatomy of an MP4
Ensuring two different users don't upload a file with the same generic name. _-4929618714516.mp4
Platforms like rename files to keep their servers organized. A name like _-4929618714516.mp4 is often a "Hash" or a unique "ID" used by a database to: Stripping away the original metadata (like your location