Packaging allows you to place a feature without fully constraining it, so that you can move it around as you please - as opposed to a fully constrained part which must have placement constraints removed prior to being able to adjust components
When you are adding a component to an assembly, you may not know where that member fits best, or you might not want to locate it relative to other geometry. You can leave such a component either partially constrained, or unconstrained. This component would be a packaged component. Although it is a part of the assembly, it is not assembled parametrically. Use packaging as a temporary means to locate the component; finalize the location of the component with assembly constraints. One way to package a component is to use the PACKAGE menu. Another way to package a component is to assemble the component, then close the Component Placement dialog box before the component is fully constrained.