hi, what is the difference between specify time out and do not specify the timeout value in the web.xml file

hi, what is the difference between specify time out and do not specify the timeout value in the web.xml file

   <session-config>
        <session-timeout>30</session-timeout>
     </session-config>
   

before, i do not specify this in the web.xml file
when the session has been timeout, i sometime got some exception

but after i did that..no exception any more..why???