Why/Can we use 'virtual' for properties in classes?

public class Employee
{
   public int EmpID { get; set; }
   public string EmpName{ get; set; }  
   public string Address { get; set; }
   public virtual ICollection<Dept> dept{ get; set; }
}

public class Dept
{
   public int DeptID{ get; set; }
   public string DeptName{ get; set; }
   public string Email { get; set; }
   public virtual Division div { get; set; }
}
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The virtual keyword has an effect when used on properties in EF Code First.

• Lazy Loading: Any virtual ICollections will be lazy-loaded unless you specifically mark them otherwise.
• More efficient change tracking. If you meet all the following requirements then your change tracking can use a more efficient method by hooking your virtual properties. From the link:

To get change tracking proxies, the basic rule is that your class must be public, non-abstract or non-sealed. Your class must also implement public virtual getters/setters for all properties that are persisted. Finally, you must declare collection based relationship navigation properties as ICollection<T> only. They cannot be a concrete implementation or another interface that derives from ICollection<T> (a difference from the Deferred Loading proxy)