If you check carefully you will see the Random Orbital Sanders (ROS) are round. The round disk spins while oscillating, making the scratches it leaves less visible.
If you see a square sander, it is a "pad" or "finishing" sander, and works much, much slower.
The ROS was developed to approach the sanding rate of a belt with quality nearing that finish quality of the pad sander.
Since I've purchased my ROS my finishing sander hasn't been out of the box, although they are better at getting into corners. That said, the ROS is so much quicker that it's work sanding before assembling to get around that!
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