Do I need to retain Previous iTunes libraries

The Previous iTunes Libraries folder contains dated copies of the iTunes database (the iTunes Library.itl file) that are created automatically during some (but not all) iTunes updates - this only happens when the update includes a change to the database structure. Unless you think that you're ever going to revert to a very old version of iTunes, you can safely delete all but the newest of these files in the Previous iTunes Libraries folder.

However, this is probably unrelated to the increased disc usage you're seeing. My main iTunes library is around 1TB; the Previous iTunes Libraries folder is around 53MB (containing the copies of the iTunes Library.itl file from the last three updates that included a database change, from October 2014, June 2015 and July 2015) . I suspect that what you're seeing is a consequence of how the library consolidation process works. When you use this function, iTunes will copy any media files that are not located in the standard iTunes Media folder location into that folder structure. The original versions of the media files, though no longer linked to the iTunes library database, stay where they are. Is it possible that you have media files/folders contained within the Previous iTunes Libraries folder? In normal operation/configuration this should only include the dated copies of the database file:

My suggestion would be to backup your complete library as it is now to a suitable device, delete everything in the Previous iTunes Libraries folder with the exception of the most recent iTunes Library yyyy-mm-dd.itl file, run iTunes and check that you can play and sync everything as expected. If so, you're good to go - if not, you can use the backup to restore the library folders to their previous state for further investigation.