Is it necessary to "Restore" files from a Time Machine backup if I can already access them directly from the backup disk using Finder?
No - you don't need to have two copies of the files you backed up.

I think it's a good idea to restore all the files you want to have a backup copy of, however. Spotlight search will let you read / search all the files, but you have no safeguard for when the backup disk dies or fails, so your retention policy for everything you don't copy back is until that disk has a problem.

Files within Backups.Backupdb are intended to be read only, so you shouldn't expect to ever make changes there (or even be able to delete them) so if you can modify them - you have a backup that's disabled or malfunctioning and making a restore so you can set up a new backup is highly advisable.