The benefits in communication gained by using a network are massive, and yet with them comes an increased security risk. The moment you open the door to such communications with other machines means people can get in as well as out, without proper security.
For this reason, a network administrator will put in place many policies and provisions to ensure only people intended to access various data can do so. This may be in the form of adding permissions or passwords (usually along with a user account and ID) to areas, encrypting data, putting firewalls in place and doing frequent checks for malware like Trojans using high performance anti-virus software, A handy side-effect of creating user IDs and passwords is that those can be grouped together, allowing different levels of user different access - for example, admins can change anything and normal users can only view files that are not their own. In addition, a network administrator may use these IDs to track movement and monitor suspicious users, or find information identifying usage trends.
There are different levels of security. A small office, or household private network may only require basic security protocols to keep banking information and transactions, and correspondences secure. On the other hand, huge multi-national companies and governments require entire divisions of employees devoted to maintaining a rigorous level of security, especially when their work involves a public network.