Not sure if you are old enough to remember the old vinyl records, where you put the needle down on a track and the record spins, and the needle tracks the groove of the record and plays the music.... that is essentially the same thing that is happening in a hard disk, or a CD / DVD drive, except in the hard drive, there is no needle, there is a magnetic head that reads / writes magnetic data, and a CD / DVD drive has a laser to read pits in disc surface. Both methods rely on the speed of the data embedded on the disk rotating under the head to achieve a high data output rate. Slow down the platters, and you slow down how fast the head can read the data.
A hard drive can not only spin the disk faster than most CD / DVD drives, but they usually have several disks and heads stacked up on top of each other inside the hard drive case. These are referred to as "platters". The more platters and the faster the rotation of the platters, then the faster you can read data. CD / DVD drives only have one platter and one head, and usually much slower spin rates (RPM).
To put this into perspective, the average hard drive spins at 7,200 RPMs, and a quality 52X CD drive spins at 10,400 RPM (not always, but on average, due to something called "constant angle velocity, the speeds have to be varied), but due to the fact that the hard drive has multiple platters and heads, its read/write speed is effectively tripled or quadrupled by the stacking of the platters. Also, most high speed CD drives never really reach the max RPM's they are capable of, because CD and DVD disc's get very unstable at RPM's reaching 10,000 or above, causing them to wobble or even shatter in the drive, so the read mechanism on the drives throttle the speeds back when the laser cannot read data reliably. Very few CD or DVD discs are made to a quality level to be able to spin up as high as 10,000 RPMs without getting seriously out of balance and wobbling in the drive.
Nothing worse than a bored electron. Let's keep them busy.