The packages provided by Unity can save you a lot of development time. They usually contain resources (such as texture maps, materials, and so on) and fully implemented features ready to go into your project. When creating a new project, Unity offers to install those packages into the Assets folder. However, if you've missed it at first, you can still import them into your project at any time.
Unity installation files include a number of packages that can be imported into your project as ready-to-use resources. Inside those packages are all the assets needed to implement a specific feature or functionality. Once imported, new features can be accessed through the Project view (and dragged and dropped into your level) or through newly added menu items (the Tree Creator package, for instance, adds the Tree option into the Create Other sub-menu of the GameObject menu).
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