Networking components will eventually interact with computers and devices outside of itself. Whereas during system development you are only interacting with your program internally.
For example, when you first work on a website you can begin working on it on your personal computer not connected to the outside world. When you are done working on your website you will now upload it to a webserver (or you will create your webserver) and now you are opening up the doors so to speak to the Internet. When you "open the door" you need to make sure the proper security is in place so that no one hacks your site. This is ussually done by making sure the webserver you are on does not have any known security vulnerbilities and has the proper firewall settings to prevent unauthorized access.
