Signed URLs are great because it allows you to allow third parties access to a file without them having to authenticate against AWS.

Our primary use case is browser-based uploads. You don't want people uploading anything and everything, like the wordpress upload folder. And it's timed, so you don't have to worry about someone recycling the URL.

I use presigned urls as part of a federation layer on top of an s3 bucket. Users make authenticated requests to my api which checks their permissions (if they have access to read/write to the specified slice of the s3 bucket), my api sends a presigned url back to allow read/write/delete to that specific portion of the bucket.