ASP.NET calls the difference between Response.Write and ClientScript.RegisterStartupScript in javascript

For example, the following code

 
StringBuilder sb = new StringBuilder(); 
sb.Append("<script language=javascript>"); 
sb.Append("alert(document.forms.length);"); 
sb.Append("</script>"); 

Response.Write(sb.ToString()); 
ClientScript.RegisterStartupScript(this.GetType(), "test", sb.ToString()); 

So obviously, Response.Write is going to get 0, ClientScript.RegisterStartupScript is going to get 1.