It can happen when you have multiple projects that you do not wish to process a certain ABAP project as an active version. In this case, you could (temporarily) close this project. The consequence of this is that the corresponding project tree is not longer visible. In addition, a closed ABAP project is no longer taken into consideration for the object search.
The project node is still visible in the Project Explorer so that you are able to re-open the project whenever you need it.