If you have a gaming machine and a laptop, and want to use steam in-home streaming, but you only have 1 internet port in your room, and you only have a switch at your disposal? And it turns out just switching everything together doesn't work because your 1 port has port security enabled?
VLAN can be used to make sure that the port-security port doesn't see the computer while still all connected to the same switch.
1 - College LAN 2 - Laptop 3 - Gaming computer
Set up a VLAN between 1,2 and 2(tagged),3(PVID set to this VLAN). Pretty standard.