Lots of reasons. I've got three hard drives. I run two identical fast (10,000rpm) drives in Raid 0, which means they act as one big hard drive. It makes for faster data retrieval - this machine is built for gaming performance. I also have an external hard drive on which I do backups and store copies of my data.
If you set up two drives in Raid 1 what you're doing is creating a mirror image of the first HDD on the second, so that if one fails, the other one can step in seamlessly. That I think is a luxury, but people do use that set-up.
Or you don't have to set them up in Raid, you could use one smallish hard drive just for the operating system and store all your data on the other one so that if the OS goes *phut* you won't lose all your data.