If you have a long session and open many folders during the session - and you have the option checked - it will re-open those folders when you start Poser. The number of open folders will increase if you open other folders in the next session. After some time you will have all folders opened in your library and Poser will open them when you start.
In a large runtime this will take a lot of time and also makes the library window contain a huge number of items which is more difficult to navigate.