The Communion Outside of Mass is as what I understand a liturgical service done primarily by the extraordinary ministers if the priest is not around, but according to my missal and some other sources it shows the priest can preside over this, if there is a priest why cant they just the celebrate the mass since the priest is already there.
He might have already celebrated his Mass for the day.
Although we don’t often experience it, priests can distribute Holy Communion in a service other-than-Mass. This might be done for any number of reasons (like the one Phemie just mentioned).
It’s not always that the priest “can’t” celebrate Mass. Maybe he could. The Church simply says to us that there might be times when the community decides that Mass won’t be scheduled for that time, and then makes provision for that by providing a liturgical service.
Think of it this way:
The Church is simply saying to us “if you have a need to distribute Holy Communion without Mass this is the ritual for doing so.”
The only time I ever experienced this was during elementary school. I was attending morning Mass and our (Catholic) school bell had just rung, but the priest was only just arriving. He did a communion service for us presumably to get us into class.
Generally speaking, if a priest is present or one is available, then Mass should be offered. Even if a priest has already said Mass that day, for pastoral reasons, he can binate and say another Mass. As far as I know, bishops generally make this approval so that priests don’t have to get permission every time this happens.
Rome has given directive regarding these larger assemblies for communion outside of Mass (there is also a shorter rite in both the OF and EF) for the faithful to receive communion outsid of Mass. It was more common prior to Vatican II for communion to be received outside of Mass because Mass was only permitted to be offered before noon. In order for those who couldn’t make it to daily Mass in the morning, but still needed to receive communion for First Fridays and First Saturdays Devotions.