VLAN Interfaces are required in network scenarios where you have different VLANs and need Inter-VLAN switching on Layer3 (Routing capable) switches. Every VLAN that needs to be routed should have a VLAN interface.
Let's say we have VLAN 10 which hosts the subnet 192.168.10.0 subnet, VLAN hosts 192.168.20.0 subnet and VLAN 30 hosts 192.168.30.0 subnet. For Inter-VLAN routing to work, we need to have a VLAN interface setup for each of these VLANs and configured with an IP address from the same subnet which will be the default Gateway for that subnet. Lets say, 192.168.10.254,192.168.20.254.192.168.30.254 are the IP addresses for VLAN Interfaces of VLAn 10,20,30 respectively.