Special mount points is a very powerful feature. You can specify a persistent home directory by mounting to a USB memory stick.
You may want to mount a nfs server to get the binaries (/usr/share) of most programs.
Here you can also specify samba servers to connect to.
The only limit on possible configurations you may face is your own imagination!
Here's another one, a "/home" over the network to a SAN where all your users home directories are stored centrally.
When you specify a special mount point, it is a typical entry into the "/etc/fstab" file.
The dialog which pop's up when you click Add will have all the options to supply the data required for an entry.