SSD's are fast but there are still pros to using RAIDs.
-Redundancy. When your SSD fails, that's it. It's done and your data is gone. If you have a RAID5 one drive can fail and it's still usable and you can rebuild. On a RAID6 two drives can fail and you still can rebuild that.
-Price/space. Larger SSDs are pretty expensive and small. When dealing with uncompressed stuff (especially at higher resolutions/framerates) a terabyte won't cut it. You can get pretty big volumes on a RAID setup pretty cheaply.