Transparent data encryption is used for:

Transparent data encryption (TDE) is used for data and log files at rest. This means that TDE encrypts the database files on the disk and decrypts them when they are read into memory. TDE protects the data from unauthorized access or theft if the physical media, such as drives or backup tapes, are stolen or lost. TDE does not encrypt data across communication channels, data currently being processed, or data in random access memory (RAM). These types of data require different encryption methods, such as SSL/TLS, column encryption, or memory encryption12.

References:

Transparent data encryption (TDE) - SQL Server | Microsoft Learn

Transparent Data Encryption - Oracle Help Center