Ease of replication.
It is easier to replicate several chunks compared to the entire file. If any error occurs during the replication, only the failed chunk needs to be copied again.
Balance server loading.
Both reading and writing operation can be separated among all chunk servers.
Enhance throughput for both reading and writing.
Both reading and writing throughput can be enhanced since hundreds of server can serve the requests simultaneously. Application gets the metadata of chunks of a file from the master server then gets those chunks from chunk servers directly.
Better of disk utilization.
If your files tend to be large then a chunk and disks have only a few space, it is easier to find enough space for a chunk rather then the entire file.
Ease of integrity check.
Compute the checksum of a chunk is faster than an entire file. When a corrupted chunk is detected, it is easier to fix the chunk instead of entire file as well.