The world has some really nice people in it. That being said, the world has some really bad people in it. Those bad people want to try to get into your server and use it for their own evil deeds. Your job as a Linux Administrator (one of your many jobs!) is to make it really-really hard for them to do that. I'd say make it impossible for them to crack, but there's no such thing. If they're determined enough, have enough resources (and probably have someone on the 'inside') they'll likely get in.
One of the tools I use often is called ConfigServer Firewall. This is a software firewall script that lets you lock down your server pretty well. It also includes LFD (Login Failure Daemon) to block out crackers with failed logins.
You can edit and set it up via command line by editing the csf.conf file directly or you can use the user interface (UI) feature. It even ties into popular control panels like cPanel.
One of the great features of this software is that you can specify a set of iptables rules that you'd like enabled on CSF startup.