How events are more robust than delegates

Consider the following example.

The Stock class fires its PriceChanged event every time the Price of the Stock changes:

public delegate void PriceChangedHandler (decimal oldPrice,decimal newPrice);
public class Stock
{
    string symbol;
    decimal price;
    public Stock (string symbol) { this.symbol = symbol; }
    public event PriceChangedHandler PriceChanged;
    public decimal Price
    {
        get { return price; }
        set
        {
            if (price == value) return; // Exit if nothing has changed
            if (PriceChanged != null) // If invocation list not empty,
            PriceChanged (price, value); // fire event.
            price = value;
        }
    }
}

If we remove the event keyword from our example so that PriceChanged becomes an ordinary delegate field, our example would give the same results. However, Stock would be less robust,

My question is in what way the event keyword makes the stock more robust?