For security reasons!
SSDs store the data scrambled all over the place and on different flash chips.
Because flash can break, they all have more storage space than advertised and useable.
Now assume you have top secret information on your disk unencrypted. You now decide that's a stupid idea and encrypt the whole drive.
But you can't encrypt the whole drive. The SSD just shows you 16GB of space, while it has 20GB internal (in reality, the additional space is less).
You encrypt all of the 16GB, but inside the drive there are still 4GB and you have no way to know what's stored there. Maybe one flash chip is even partially defective and the drive will never touch it again. A data thief could still directly read data from that.
Another reason is to allow fast data destruction. If you have to erase a 1TB SSD with 400MB/s, that will take 42 minutes. If you want to remote-wipe your SSD in a stolen laptop, in this 42m the thief will see that something is wrong and cut the power. For the same reason, most newer smartphones are encrypted by default, even if you don't need any pin.
Wiping a encrypted SSD/phone works by just wiping the 128bit (or 256bit) key. After that, all the data is worthless.. This takes less than a second.
4Please edit your question to add some details on the OS this SSD was running on as well as the exact make/model of the SSD. To my knowledge, unless you have full disk encryption enabled or the drive has native encryption, it all sounds like a ruse by the data recovery company to deflect their incapability to some “magic” that prevented them from recovering data. SSDs should just be storing data in a raw format—like all other storage devices—and not any encrypted format by default. – JakeGould – 2015-10-14T04:32:34.300
1some external drives do have hardware encryption by default but I'm not sure if an internal one also encrypts data – phuclv – 2015-10-14T04:39:25.893
8@JakeGould many modern drives are internally encrypted. Annoyingly, finding sources is a pain when you actually need them, but at the very least sandforce's controllers certainly does this. – Journeyman Geek – 2015-10-14T04:52:25.210
@JourneymanGeek SSD or hard disk drive as well? It seems this is a functionality that is mainly borne in the needs of SSD drives that might have migrated to newer hard disk drives as well? – JakeGould – 2015-10-14T05:17:23.433
3
@JakeGould: Many newer SSD controllers, including all recent SandForce and Samsung controllers, use always-on encryption. Some newer Seagate hard drives, including several consumer desktop models, are self-encrypting (see here; this is what enables the Instant Secure Erase feature). Most newer WD external hard drives are self-encrypting.
– bwDraco – 2015-10-14T05:22:26.6271SSDs encrypt or at the very least scramble the data (citation needed here!) for wear levelling reasons. HDDs have no need to encrypt data at a firmware level for the same reasons. – Journeyman Geek – 2015-10-14T05:34:36.130