They are an active history of your commander, data, exploration, assets. Each time you login and play, a journal file is written.
Things like EDSM, EDDiscovery, and Frontier's API use them and push data from them to third party sites.
So, I have journal files from the day I started playing (2014) and my history on all third-party sites would be an accurate history of my game play.
If you archive most of them, or remove them from the folder in which they live, i.e. "Documents\Saved Games\Frontier Developments\Elite Dangerous" and only left the last month or so, then only those journals would be used for third party sites. As long as the journals you now archive have been pushed to all third-party sites you use, then the third-party sites would have complete history, even though you are only using the last thirty days.
I moved my "Documents\Saved Games\Frontier Developments\Elite Dangerous" directory to a different drive where I have more storage, so the space they take does not affect my system. You might want to move the "Documents\Saved Games" directory to another drive, as the "Saved Games" directory (where journals are stored?) is also used by other game saves and they may want more storage space as well.
