Imagine a torrent client (or http client downloading a file in parallel using HTTP range requests). It creates an empty file and then it has downloaded a 1MB of data to write at offset 100GB and wants to write it to disk. It does not want to pay the price of waiting for 100GB of zeroes to be written. The other blocks will all be downloaded and written eventually, all out of order. If the filesystem had an atomic operation to transform a bunch of (block aligned) files into a single file (like AWS S3 Multipart Upload), then sparse files would not be needed for this case.