What's the purpose of uniqueId in FileWriterBucket

[wu shaoj]

Hi, folks,

>From 1.14, file sink add a uid to a committed file, so would you mind to tell 
>me what’s the purpose of this field? Can it be removed safely?
https://nightlies.apache.org/flink/flink-docs-release-1.14/docs/connectors/datastream/file_sink/#part-file-configuration