The short version is that our data, log and backup files are encrypted at rest (i.e., on the storage layer), so that an attacker cannot simply copy and attach the data and log files, or restore a backup, without having access to the master key. If backup tapes or drives are stolen, the data on those devices cannot be recovered.
We can also use what is known as a Hardware Security Module (HSM) to provide keys to secure the database. This is a dedicated physical or virtual device, separate from SQL Server, that generates keys for various services in an organization.
Read on for Randolph’s argument. For the counter-argument, check out Simon McAuliffe’s article on the topic. Over the past couple of years, I’ve grown much more sympathetic to the idea that TDE’s primary use case is in weeding out the rabble-rousers with nary a clue, but it’s not very helpful against a knowledgeable attacker with administrative access to your SQL Server.