1 - In general yes, although if you only recently migrated to something post-Mojave you might back them up elsewhere or keep them for a little while at least. See Managing your Mac media libraries - Apple Community for more on the cruft that might be left over from an old iTunes Library and tip on rearranging things into the default layout for the new apps.
2 - Randomly, when Apple decide that an update to the apps required an updated library schema. Libraries are forwards compatible, and when this happens the old database is archived in the Previous Libraries folder and then used to construct the new version of it in the new format. A possible way to recover, for example, a playlist deleted in error, but safer yet is to ensure your Music folder is included in Time Machine or some other regular and systematic backup process.
tt2