Swap can be used to write parts of RAM to disk. This is disabled as it could write important information such as passwords, encryption keys etc. to disk.

It's disabled by setting vm.swappiness=0 in /etc/sysctl.d/swappiness.conf.

/usr/bin/swapon is also replaced with a dummy binary that just runs /bin/true to prevent something from mounting a swap partition on the hard drive.