This tool restores sparseness lost after you’ve created and deleted files. It works for virtually any guest OS, and supports almost any filesystem.
It can also do some clever things like finding sparseness in the unused parts of LVM physical volumes, or converting between formats (eg. raw to thin-provisioned qcow2).
It’s also (like virt-resize) a temporary tool: hopefully one day KVM and guests will be able to resize and trim themselves. That day isn’t here yet.