The man page tells us: reduces fragmentation of extent based file.
The file targeted by e4defrag is created on ext4 filesystem made with "-O extent" option (see mke2fs(8)). The targeted file gets more contiguous blocks and improves the file access speed.
Let's try this tool.
You can try under root account this:
# e4defrag -test -cv /home/your_user/
The final result on my account is :
Total/best extents 170276/165520 Fragmentation ratio 0.03% Fragmentation score 0.24 [0-30 no problem: 31-55 a little bit fragmented: 55- needs defrag]
Also, you have to run e4defrag with the -test parameter for now, since it isn't a fully tested version.