When would one want to use Oracle transparent data encryption Oracle TDE features comes with various limitations such as not being able to encrypt columns which are used in foreign key constraints. And probably is available only with enterprise edition which costs upwards of 100 thousand dollars. 

Why would one want to pay and use TDE instead of simply using file system encryption with the OS? This is free and byepasses various limitations regarding FKs etc.

Am I missing some advantages that it provides?
Transparent data encryption guarantees the data is always encrypted at rest, implying even the backups are encrypted, no matter where they are physically stored, be it tape, disk, a DVD, etc. 

Encrypting data at rest is a great way to control who sees the data, even if someone misplaced an entire copy of the database or a steals a database backup.