why does pppoe connection has session timeout ?

hi,
 
maybe this is not a right place to ask this question,  if intrusive accept my applogy please.
 
Doing with those access network ,   there is always a max session timeout  for each pppoe or dial-up link.
According to document of broadband access server, a default max session timeout value is designed.
 
In radius protocol document, there is also max session timeout attribute.
 
To my experience, no matter how we do  dialup connection (  ppp over fiber or ppp over telephone network)
internet connection will has a max session time.   no matter how we do , after that time  ppp connection will be 
cut by access server.
 
so why  does protocol designer set this parameter ?
 
Jing 
 



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