All UNICES, and operating systems similar to UNIX like to be informed when filesystems are added or removed. The operating system can improve performance quite a bit by being able to ``cache'' directory structures, by being allowed to read more than requested, and to delay writing to disk.
In general, only the superuser is allowed to mount (add) or umount (remove) filesystems. It is possible to set things up so that ``portable'' sources of information, like floppy disks and CD-ROMs can be added and removed by ordinary users.
Mount issued without any arguments, will list out all the filesystems currently mounted.