Does the speed of sanding a piece of wood affect the end result? In particular, what is the point of having a dual speed random orbital sander?
A lower speed can be desirable if you're sanding between thin coats of finish or thin veneers, and you don't want to sand through the current coat or the veneer. But many of the lower-end or homeowner-grade random orbit sanders aren't aggressive enough for it to matter.
As with many tools, lower speeds on a sander offer more control, so you may avoid rounding off corners or having the sander skip or jump and gouge the material (particularly if you're using a more aggressive sander).
Some tools have sanding attachments which only work well at lower speeds. For example, if you use an oscillating multi-tool for sanding at the tool's higher speeds, the hooks for the hook-and-loop backing pad will melt, rendering the attachment useless.
Variable speed sanders are most useful for heat sensitive materials, which wood is not. (At least not to the range of temperatures that sanding will generally expose it too!)
A variable speed sander can help control the amount of material removed if you're just trying to make a light pass though.
Actual scorching of wood is possible with a belt sander running at a high enough speed and with the wood held tightly against the belt (particularly a problem with worn belts). That's the most obvious 'effect' resulting from speed that I can think of sanding wood itself and it's a flaw in sanding technique and not directly a fault of the sander. But other than that the sanded surface created with grit X is the same whether achieved at high or low speed — the scratch pattern is identical as far as I can tell.
Re. random-orbit sanders though I would expect that a slower speed would be offered for two reasons: to help reduce the tendency towards corning (see previous Question) with resinous woods and when sanding finish, but primarily to help make sanding more controllable, e.g when sanding thin veneers, near edges or corners, or when sanding a finish.
There are multiple issues sanding finish, the most obvious being that it's very easy to inadvertently sand through to the bare wood, but also many finishes are heat-sensitive so higher sanding speeds can sort of melt their surface, leading to corning as well as a sub-par sanded finish.
In addition to the other comments: sanding produces heat. Depending on what you're doing, it may be possible to produce enough heat to affect the wood's color, "burning" the surface a bit. That's more likely with cutting tools, but a really aggressive sander could do it.
But, yeah, mostly it's a matter of getting the job done quickly traded off against the risk of losing control and taking more than you intended or sanding irregularly.