Vlan interface on hardware / virtual switch

Hi guys,

I have configured a hardware switch made up of 4 physical interfaces. 

Then I noticed I can configure a VLAN interface and use the switch as support/base interface (for the VLAN interface).

What's the logic behind it and what is the use case?

On my scenario: each port that belongs on the switch will act as an access vlan port or like a trunk vlan port (tagging traffic). 

I went on both config system interface and config system virtual-switch but could not find any tag related option.

Thanks,

Florin.