My understanding is as follows:

When hot backups of tablespace datafiles are taken any updates continue as normal except file header blocks are not updated for checkpoints. When the backup is complete the header is updated for the last checkpoint so that the file is consistent with that point in time.

As the OS copy may be inconsistent (as you may have made changes to the blocks) they require the archived logs to be applied as part of the recovery process to ensure they include all the changes missed during the begin and end stages of the backup process.

Regards