C# Difference between firing event with invoke and without

Question

I encountered a code which made me wonder, why in this case firing event is with Invoke and what is the difference between two ways of firing the event:

public event EventHandler<NewUsersFrameEventArgs> NewUsersFrame;
protected void OnNewUsersFrame(List<ZigInputUser> users)
{
    if (null != NewUsersFrame)
    {
        NewUsersFrame.Invoke(this, new NewUsersFrameEventArgs(users));
    }
}

and

public event EventHandler<NewUsersFrameEventArgs> NewUsersFrame;
protected void OnNewUsersFrame(List<ZigInputUser> users)
{
    if (null != NewUsersFrame)
    {
        NewUsersFrame(this, new NewUsersFrameEventArgs(users));
    }
}