Large corporations have several points of access that can be exploited by parties with malicious intent. Protecting the various points of access to your network is the key to any successful IDS installation. You're only as strong as your weakest link.
Intrusions coming in from the Internet are important to combat, but misuse and intrusions attempted through the extranets or inside the corporate network are equally as critical to defend against. In fact, research statistics show that insiders are the most common source of attacks.