Raise and Consume an Event in C#

Hello,

Please see the example 2 in the below link:
https://learn.microsoft.com/en-us/dotnet/standard/events/how-to-raise-and-consume-events

We can rewrite this example and remove the OnThresholdReached method.

I rewrite example 2, without that method and works fine.

public class Counter  
{  
    private int threshold;  
    private int total;  
    public Counter(int passedThreshold)  
    {  
        threshold = passedThreshold;  
    }  
    public void Add(int _value)  
    {  
        total += _value;  
        if (total >= threshold)  
        {  
            ThresholdReachedEventsArgs args = new();  
            args.Threshold = total;  
            args.TimeReached = DateTime.Now;  
            handler(this, args);  
        }  
    }  
    public event EventHandler<ThresholdReachedEventsArgs> handler;  
}  
  
public class ThresholdReachedEventsArgs : EventArgs  
{  
    public int Threshold { get; set; }  
    public DateTime TimeReached { get; set; }  
}  

I have three question here:

1- Why should we write OnThresholdReached method?
2- Why this method should be protected?
3- Why this method should be vitrual?

Thank you