The sincedb is an in-memory database that records which files logstash has read and how much of each file it has read, so that if data is appended to the file logstash knows to consume it.
sincedb_path is used to persist the in-memory database across restarts. That means that if you set sincedb_path to NUL (on Windows, or /dev/null on UNIX) then when logstash is restarted it thinks every file is new and will re-read it.