What is the point of a reuse identifier for annotations

I am trying to implement annotation onto my mapkit I am using

mapView.register(EventMarkerView.self, forAnnotationViewWithReuseIdentifier: MKMapViewDefaultAnnotationViewReuseIdentifier)

However, I often see this sort of format

    func mapView(_ mapView: MKMapView, viewFor annotation: MKAnnotation) -> MKAnnotationView? {
    guard let annotation = annotation as? Event else { return nil }

    let identifier = "marker"
    var view: MKMarkerAnnotationView

    if let dequeuedView = mapView.dequeueReusableAnnotationView(withIdentifier: identifier)
        as? MKMarkerAnnotationView {
        dequeuedView.annotation = annotation
        view = dequeuedView
    } else {
        view = MKMarkerAnnotationView(annotation: annotation, reuseIdentifier: identifier)
        view.canShowCallout = true
        view.calloutOffset = CGPoint(x: -5, y: 5)
        view.rightCalloutAccessoryView = UIButton(type: .detailDisclosure)
    }
    return view
}

Would I use the latter form if I had more than 1 type of annotation?

I am also curious if my MKMapViewDefaultAnnotationViewReuseIdentifier is valid? I did not choose it, but since I only have 1 annotation type, it shouldn't matter, correct?

Finally, does the reuse identifier still have the functionality of the dequeueReusableAnnotationView(withIdentifier: identifier)?

There are many things to consider here:

• MKMapViewDefaultAnnotationViewReuseIdentifier is only supported on iOS 11+, so if you want to target an earlier iOS you might want to use the latter form.
• Something similar to the latter form is good to implement not only if you have more than 1 type of annotation; the point of using reuse identifiers is that whenever an annotation goes off the screen it can be reused in a different annotation, this saves a lot of resources sometimes.

Bottom line:

if only targeting iOS 11+ and only have 1 annotation you are fine with MKMapViewDefaultAnnotationViewReuseIdentifier. Otherwise consider the other form.

Hope this helps you make a decision.