Many NetBeans users have different areas of work in the IDE. For example, they might have some Java EE projects for a web application for their employer, but work on some personal Java SE projects in the evening. NetBeans IDE lets you open and close projects at any time, but if there are a lot of projects in each group this may be cumbersome and error-prone. Switching user directories is an unreasonable workaround to this issue. Therefore, a number of users have requested that the IDE provide some facility for them to group projects together to make them easier to open and close as a unit.
The feature is described in the screencast below. Note that it has no sound.