There are multiple use cases for mergerfs, most commonly for mixing
multiple (hetrogeneous) file systems into one single union.

A good example is unifying your /tmp over tmpfs and physical block.

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tmpfs   /ram-tmp        tmpfs   defaults        0       0

/ram-tmp:/media/SSHD/tmp-disk:/disk-tmp      /tmp    fuse.mergerfs   x-systemd.requires=/media/SSHD,x-systemd.requires=/ram-tmp,defaults,use_ino,allow_other,default_permissions,nonempty,minfreespace=1G,category.create=all    0       0
====================

* In the above example, we first create a tmpfs based file system
* We also have 2 block based file systems: /media/SSHD/tmp-disk and
/disk-tmp
* Using mergerfs, we create a unified output, exported over /tmp


This allows for a fast and efficient tmpfs based /tmp, while at the
same time helps you from running out of file system space, if your
requirement is beyond the limit of tmpfs size allocated for /tmp
