Why do wee need computer network

You do not need a network unless there is a need to have more than one host communicate with each other.

Just to expand on what JorgeM is saying,

A computer network is designed to share information and/or data between two or more computers. This isn't a requirement, and a computer that isn't part of a network is referred to as a "stand alone" or "independant" system. These types of systems can have advantages in that they are theoretically impossible to attack unless you are physically next to it, and they are often cheaper to maintain as you don't need to manage a network infrastructure around it.

On the other hand, it means that the only way to share information with it is via a physical storage medium (this includes updates to software installed on it etc.).
Computer networks therefore have a significant advantage, and without them the world we live in would be very different.

You would have no internet (the largest computer network in the world), a major and highly valuable resource in the modern personal, and in the business world.