They duplicate files to reduce load times. Here's how Arrowhead Game Studios themselves tell it:

https://www.arrowheadgamestudios.com/2025/10/helldivers-2-te...

The post stated that it was believed duplication improved loading times on computers with HDDs rather than SSDs

The idea is to duplicate assets so loading a "level" is just sequential reading from the file system. It's required on optical media and can be very useful on spinning disks too. On SSDs it's insane. The logic should've been the other way around. Do a speed test on start an offer to "optimise for spinning media" if the performance metrics look like it would help.

If the game was ~20GB instead of ~150GB almost no player with the required CPU+GPU+RAM combination would be forced to put it on a HDD instead of a SSD.