A single request is useful; dozens of organised, reusable requests are powerful. Postman's collections, variables, and environments are what take you from ad-hoc calls to a maintainable test suite.
Collections
A collection is a saved group of related requests — for example, all the calls for a "users" API. Collections can be organised into folders, shared with a team, and run as a whole.
Variables
Variables let you avoid hard-coding values. Instead of typing a base URL or token into every request, you reference a variable like {{baseUrl}} and change it in one place.
Environments
An environment is a set of variable values for a particular context — dev, test, or production. Switching environments re-points your whole collection without editing a single request.
One collection, many environments
Conclusion
Collections group your requests, variables remove hard-coded values, and environments let one suite run anywhere. Together they turn scattered calls into an organised, reusable foundation for automation.