Time Machine is an archive rather than a backup, because it retains multiple versions of files as you change them. That’s useful, because if you’re editing an image, a word-processing file, or another document, you can use Time Machine to restore a version before you made certain changes.
It always maintains at least one copy of every file, no matter how many times it’s revised, and if Time Machine has just a single version, it’s always the latest copy. If a file is never changed from its initial backup, Time Machine never deletes it, either.
Apple has Time Machine automatically consolidate snapshots. It’s set to run every hour, making a copy of any file changed from the previous 24 hours. After 24 hours, Time Machine deletes hourly backups, but keeps one per day. After a month, it deletes daily backups, but retains one weekly snapshot.