Dariusz,
in a nutshell, the purpose of the logs is to hold information about Changes in the Database (=transactions). This is needed to recreate the changes after a Database crash. The logs are only kept from the last backup on, as the database can be restored to this point using restore. So this is the reason and logic for deleting the logs after a backup.
I do not know If you can choose to keep the logs.
BTW, what do you need the logs for?
regards,
Timm
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