Before a computer can make use of a storage device, this device must be mounted on the computer's own file system. The location where the device is mounted is known as the mount point. To access the device contents, one needs only to access the mount point.
Mounting a device ensures that the computer recognizes the device file system and can make use of it. If the computer cannot understand the file system format, it will refuse to mount the device. Because file system formats vary according to the type of devices (or sometimes the operating systems in which these devices were formatted), the mount operation is an all-or-nothing guarantee that data will be well understood and reading or writing operations will preserve the file system integrity.
For more information:
Mount (computing), at Wikipedia
Mount, Linux Man Page
Mountvol, a little known Windows XP mount command