Eraser *could* regain some disk space under certain circumstanes with an NTFS-formatted drive, at least according to my understanding...
When Windows runs low on hard disk space, it may decide to place a file in the disk area reserved for the Master File Table. As I understand it, even if that "resident" file in the MFT space gets deleted by normal means, the section of the reserved MFT space that was used by the file is still marked as unavailable for use. When Eraser does a free space wipe and cleans the MFT, the space that was occupied by the deleted resident file is also wiped, which the operating system then sees as "available" again (albiet still reserved for the Master File Table); for this reason the operating system reports a little bit more free space on the drive – which, again, is reserved free space, so obviously nothing to get very excited over.
In any case, it should be made clear that Eraser is not a "disk cleanup" program as one would use to clear junk internet or temporary files. Eraser's purpose is to permanently erase existing or insecurely deleted data.
Eraser can't reclaim MFT space used by a resident file that's still in the file system, i.e. not "deleted", but JkDefrag is able to move such a file out of the MFT zone if needed.