Why must I use firewall ?

[Supote Leelasupphakorn]

Hi, all

   So far, I known firewall is a choice when I want 
to protect my boxes from crackers but my question is
if I closed the service I don't use (such as port 25
for STMP) so the cracker out there can't attack,
what's the reason "firewall" come to play ?

Thanks in advance,

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