I think you might be a little confused. The Velociraptor isn't an SSD, it's a high-speed hard drive.
The benefits of a SSD (Solid State Disk) are many but the price is usually the main factor for people looking to buy one.
SSDs have: faster read/write times
low heat generation
low power requirements
The reason for this is that unlike regular hard drives that have platters (physical disks) that have to spin up, SSDs have no moving parts. Due to that fact they produce no noise (due to no vibrations), are more durable, and are capable of far faster speeds than normal hard drives.
The Velociraptor is a normal hard drive such as the one you just bought, the main difference being the rpm that the platters move at. A regular hard drive has an rpm of 7.200. The raptor has an rpm of 10,000.
The most common speeds for hard drives are 5,400; 7,200; 10,000; and 15,000rpm.
It's not necessary to have an SSD over a regular hard drive as both do the same thing: store data. But if you want only the most powerful, fastest, and most durable hard drives than SSDs are the way to go.
However they are usually much more expensive than regular hard drives for the same capacity.
A 250Gb SSD ranges in price from $600 - $850; a same capacity regular hard drive is around $50 - $55.
Hope that helps some.