getting values from control in update panel

i have a hidden control in update panel with a OK  button

on OK button currently i have a  called  a client side script  whic has button1 click event with server side ( this procedure was foloowed for doing something else)

1) whts the difference between using directly serverside click event on OK button & using as above

2) currently the values from the hidden controls gets appended even after having the postback

 <asp:Button runat="server" ID="btnok" Text="OK" OnClientClick="setvalues()"  />

function setvalues()
        {
           
        
         
          document.getElementById('<%=button1.ClientID %>').click();      
         
       
       }
doesnot the controls in update panel gets cleared in postback??