What are Piwik archive tables?

I noticed piwik_archive_blob_YYYY_MM and piwik_archive_numeric_YYYY_MM being created each month.
Can someone tell me what they are used for?

I assume it is for some archiving but I’m not sure if the data is being moved from log_visit and log_visit_action.
I am asking b/c we have some queries that are working directly with the piwik tables.


These tables contain archived data, but nothing is moved from other tables.