I see some models of random-orbit sanders have a variable speed control. Under what circumstances would I need variable speed while sanding? It's not clear to me what benefit it offers.
When you want to sand more aggressively, you use a faster speed. When you want to sand more gently (to take greater care not to sand through veneer or finish for example) you would drop to a lower speed1.
Just to note, final sanding of wooden surfaces should normally be done by hand to help reduce the chance of 'pigtails' or sanding swirls2 and to ensure the best finishing result. This is commonly done using the same grit as the final ROS sanding (e.g. 180-240) with the abrasive backed by a sanding block and following the grain. Alternatively you could do a final scraping, but if you're scraping you might skip the ROS entirely!3
1 Coarser paper and finer paper would normally be used in each case as well of course, but using the same grit there's quite a difference in aggressiveness between a high speed and a lower speed.
2 The random orbit and good technique (do not press hard, move the sander slowly) already reduce this tendency significantly over normal rotary sanders, but it's still quite easy to leave occasional swirls or some indication of cross-grain sanding.
3 See previous Answer for why scraping is better than sanding.