Oracle Database Discussions

Hi,
Regarding to this, i have read that:

"All data you create in this tablespace will be encrypted using an AES256 encryption key. You cannot encrypt an existing tablespace. To encrypt data, first create an encrypted tablespace, then use alter table move, CTAS or datapump import to move your data into the encrypted space. Remember to drop the old tablespace BUT not including datafiles. Use an OS schred program to remove the old datafile. If you are on ASM you may use the including datafiles option since you can’t schred files from the OS inside an ASM instance."

But i want to know why we should NOT drop the including datafiles, when dropping tablespace (so 'drop tablespace my_tbs including contents and datafiles'). So what option should we use when dropping tablespace?
Why we should use OS capabilities to remove the datafiles?
What happens if i remove the datafile when i drop the tablespace?

Regards,