Defrag does not delete anything..........
However fragmented files are de-fragmented and restored to preferred working order.
In the context of administering computer systems, defragmentation is a process that reduces the amount of fragmentation in file systems. It does this by physically reorganizing the contents of the disk to store the pieces of each file close together and contiguously. It also attempts to create larger regions of free space using compaction to impede the return of fragmentation. Some defragmenters also try to keep smaller files within a single directory together, as they are often accessed in sequence.