/mnt is the conventional location for temporarily mounting filesystems by hand. There is no daemon managing it and no automatic population — a sysadmin mounts something here, uses it, and unmounts it. It is a scratch space for one-off mounts: a USB drive being inspected, a disk image being repaired, another OS partition being accessed for recovery.

# mount a disk partition and browse it
mount /dev/sdb1 /mnt
ls /mnt
umount /mnt
 
# mount a disk image
mount -o loop disk.img /mnt
 
# mount a network share
mount -t nfs 192.168.1.10:/exports/data /mnt

The convention is to use /mnt for short-lived mounts managed by a person, /media for removable media mounts managed automatically by the desktop environment (USB drives, optical discs), and dedicated mount points under / for permanent system mounts defined in /etc/fstab. Some distributions create subdirectories under /mnt as permanent stubs (/mnt/usb, /mnt/backup) to make intent clearer, but this is a local convention rather than a standard.