dropdownlist id in mvc

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MVC3

@Html.DropDownList("Designation"

                      , new SelectList(ViewBag.DesignationList, "Key", "Value")

                      , "--- Select ---"

                      , new { id="ddlDesigantion", Class = "ddlStyle" }

                  )

above code DropDownList ,first param 'Designation' we can use the id of this control.
but in above code writen html attributes new { id="ddlDesigantion", Class = "ddlStyle" }.
these html attributes id or class only using for dropdownlist event .

can you please explain why this.