AFIK That wee barb should be a solid barb rather than a through hole bored fitting, and what you do is you put the vacuum line from the EGR valve onto it to plug the line and maintain the vacuum. When you disconnect the EGR, if you leave the vacuum line open to the atmosphere, there is a risk of losing vacuum and losing the vacuum at your brake pedal's servo resulting in a loss in braking performance.
Edit: If it's hollow, it's for a boost gauge, and a hose + screw will plug it, but you still need to secure the vacuum line from the EGR vacuum solenoid valve to the EGR valve that this spool piece removes.