Windy:
For the most part they're picked up by "brickers".... individuals or groups that will take out a VERY large sum of notes from one or more institutions.
Brickers will generally do this to improve their odds at getting solids, ladders and believe it or not they're very helpful with the research on different front plate and back plate number combinations, How different prefixes have been printed as well as replacements.
In the past many notes that are numbered from 01 - 100 are saved and either used in display sets or given to dignitaries.