If you enable ‘Purge old visitors logs‘ in the Settings > Privacy menu, Piwik will regularly delete your Visitor Logs from the Database. The Logs are used by Piwik to process Reports such as “Top Keywords”, “Top pages”, etc. If some old logs are deleted from your database, but reports have not been yet processed, the data will be lost forever.
We highly recommend that before deleting old visitor logs from your database,

1) you setup the automatic archive processing
2) and run the archive.php script until completion at least one time

This will ensure that all daily reports for each website have been pre-processed and the logs are no longer needed for these days.