If you have had important information UNENCRYPTED on the disk before, and you do not want any body to be able to get to it, then you will use the SecureErase feature, which will destroy all the old data.
If it was just a hard drive with some non important data, then creating a new Encrypted partition just like that will slowly overwrite the old data with the newly encrypted data.
You refer to SSD's - Yes unfortunately using secureErase will not erase all the blocks, but this does not mean it is easy to get them out. The process to get those blocks usually means taking the CHIPS off the SSD, copying each chip with a special device, and then it all needs to be put together, using reverse technology, and it is not always possible to re-assemble the puzzle, because of the way that SSD shuffles areas around to reduce read/write stress.
Simply putting it. No software will be able to read the UN-Erased parts of the SSD after secure Erase, because ATA commands forbid access to unmapped parts of HDD's