why does dail-up or pppoe access always has session-timeout ?

[Joe <sj_hznm () hotmail com>]

hi.
  a question  obsessed me for a long time.  "why my  pppoe connection to internet has a max session time, even if every 
thing goes ok? "  
  In our DSL access network , max session timeout is set to 4 days, this parameter is sent to BAS by radius server 
after finishing authenticating procedure.  As I know,   beside us some other service providers also applied this 
parameter to pppoe session, the parameter varies from   48 hours to 96 hours.   Reading  documents of BAS, we found 
this is default value for session-time on BAS, that means even if    radius server does not response BAS with  
session-timeout attribute  BAS will cut pppoe session after sometime.    so , why does those BAS designer or  protocol 
designer  set such a parameter for  pppoe access ?
should anyone do me a favor on explaining  this ? 
regards
Joe