Follow up from comments on another question.

Is there any reason as to why you might install yourself as a root CA on your own network? The only reason I can think of is forcing computers in the network to trust your own self signed certificates instead of getting them digitally signed by a 3rd party CA.

A custom CA is required if you want to use https on your corporate intranet. 3rd party CAs can only give you certificates for public domains. They won't give you certificates for intranet.local or any other hostnames which are only routed in your own network. So when you want to have a certificate for your intranet or for the web interface of your own servers, you need to add a custom CA and sign it yourself.

You might ask yourself "Why would I use https on my own network"? There is currently a trend to implement everything as a browser-based application. This means more and more processes are handled through internal web interfaces, including sensitive ones. For example, our company is currently implementing a web-based intranet application to view our own payslips (and hopefully only our own). But in larger organizations, the internal network can become so large and complex that you can't really speak of a private LAN anymore. Eavesdropping from internal attackers becomes a possibility which can not be handwaved away, so internal encryption becomes a reasonable precaution.
