You cannot symlink to files outsides a chroot. So, if you want to create a chroot jail without hard links, then you'd have to copy all files you need inside the chroot, effectively duplicating those files.
However, with hard links, you only need one instance of a file on disk, which saves space.
Note that hard linking from inside a chroot to main system files (such as /bin/bash) is not a very smart thing to do, as chrooted users can then modify exactly the files you wanted to prevent them from modifying. So you always need two copies of a file.
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