Dovecot uses several binary index and log files in order to improve
performance for accessing mails. For some mailbox formats, such as sdbox
and mdbox, the index files are part of the format itself.
For details about index files, see also: http://wiki2.dovecot.org/IndexFiles
doveadm dump is used to show the contents of those mailbox index/log files, in human
readable format. This is mainly useful for Dovecot developers when
debugging some problem.