SharePoint sites can proliferate like weeds. SharePoint gets cluttered and space becomes a concern. A site decommission plan is important if your SharePoint environment is active.
For example, with a Project Site, export task lists and calendar items to Excel workbooks. Remove items you don’t need before finalizing each file. Create an Archive library on your team site or other location, and move the Excel workbooks and other documents there. Now, you can delete the site. Good naming conventions and metatags will make it easy to find these files again, should you need to reference them.