What benefit does a Firewall bring to the system?

I have a Linux system that I want to secure. Only three people use the system, and they are all trusted.

I can close down all non-essential ports, and make everyone login by ssh instead of telnet.

If the only services still running on the system are ssh, SFTP, http and https, what more benefit can I get by paying my ISP for its firewall service?

First of all a firewall is for the people you dont know not the people you do know, if youve got all your ports shut off for good then your prob. ok, but there are many backdoor scripts etc, that can get onto your system and open up any port it wants to, thats where a firewall would come into play because it would restrict those ports if they somehow got open, also firewalls can be a valuable tool in preventing different types of attacks.

All in all if the firewall services isnt expensive, go for it, some day it will help you, but having a firewall is only the first step in security, there are many other aspects you should be more concerned with.

Well said Cbaker, a firewall is like a traffic cop. It tells people what they can do and not do and where they can do it. It prevents unwanted people from coming into your network. However, a firewall can also prevent known people from doing other things as well. like if you don't want people going out to the internet and downloading software like napster, you can block that. A firewall is almost important in any form, even home users because hackers can hack into your home system and then from there go out and hack another bigger system. And guess who just hacked the government!! Your system. Get the idea. You can use a simple linux box as a firewall. You don't have to go out and spend lots of money for it. Although larger companies do. Also monitoring is a very important aspect of it. Having a firewall is just the first step. You have to be able to catch when things are happening on your system, that's why we have companies who monitor network systems. I can go on and on but I think you get the idea. Hope that helps.