Sandbox is a copy of your production organization. You can create multiple copies of your organization in separate environments for different purposes such as development, testing, and training, without compromising the data and applications in your production organization.

Sandboxes are completely isolated from your Salesforce production organization, so operations you perform in your sandboxes do not affect your Salesforce production organization and vice versa.

Generally in each phase of the project requires different environments like during construction phase there are chances to multiple teams will work on development in this case each team requires their own sandboxes for development. After the construction period, we need one common testing environment, during the training period training team requires a separate environment for training purpose and before going to production one STAGING environment required. So for each phase different environments are required.

To understand this see the following SDLC diagram.