[1] https://archlinux.org/news/moving-to-zstandard-images-by-default-on-mkinitcpio/

This isn't really necessary as /swap is mode 0700, but a warning is
logged in the journal[1], so let's set /swap/swapfile to mode 0600 and
get rid of the warning.

[1] swapon[189]: swapon: /swap/swapfile: insecure permissions 0644, 0600 suggested.

It improves readability according to some..