Question: Why do hard disk drives mostly have the same speed? They are mostly 5400, 7200, 10000 RPM. Why not 6000 or 8000? Is there any reason or benefit for having those speeds?
Answer: The spindle rotation speed of a hard disk depends on a spindle motor, naturally. Since there are only very few manufacturers of these motors, the available rotational range is limited. In fact, the current global shortage of hard disks is because of two flooded hard disk motor factories in Thailand.
The most common rotational speed was 3600 rpm in the 1980s. This is because oscillators and motors that are based around 60-hz are very common. Standard North American AC mains frequency is 60hz. 60hz translates to 3600 RPMs. Therefore 3600 rpm AC motors are widely available. Naturally, the designs that followed were based on the same rotational speeds/frequencies.
So the most common speeds ever used are 3600, 3600*1.5=5400, 3600*2=7200. Faster drives use 10,000 and 15,000 rotational speed, apparently they use different frequencies.
This StorageReview article discusses spin speed in depth.