In normal conditions (multiple tagged traffic is expected on a trunk port, we shouldn't plug an untagged traffic into a trunk port), why does trunk need a vlan id of itself, and what's the point of tagging on trunk port?
The VLAN ID separates the traffic - you need to tag frames to indicate which VLAN they belong to when transmitting over a cable.
On many devices you can also forward untagged frames through a VLAN trunk. They will be assigned to the untagged VLAN on the receiving port.