Grinding wheels and buffing/polishing wheels you buy at the home improvement store have different max Surface Feet Per Minute needs. For buffing you generally need to get it up to 7500 SFPM otherwise you are going to be standing there all day long. Grinding wheels on the other hand generally require a lot slower speeds for many applications. For example if all you are doing is knocking a burr off a bolt you can use a coarse wheel at the highest speed no problem. If however you are using a fine grained grinding wheel to surface finish and add luster then you want to run it a lot slower. Wire wheeling (generally used for rust removal) is also safer at slower speeds as cheap wire wheels tend to throw wires at higher speeds.
Buffing and wire brushing also often requires the entire wheel to be exposed while it is dangerous to fully expose grinding wheels and stand in the plane of the wheel since if the wheel fragments at high speed you may be seriously injured.
I have noticed for example the trend at the Cheap Chinese Tool sellers to reduce inventory and stock of larger grinders in favor of smaller lower priced grinders. This is because the average casual tool user sees the RPM figure on the small grinder and thinks that's all that matters The CCT on the other hand is more worried about an idiot buying a big hefty motor and attaching a large abrasive wheel to it and having it frag him. These smaller grinders the CCT's sell are generally terribly slow and inefficient to use due to their safer and lower SFPM speeds but it's very difficult to exceed maximum safe SFPM speeds with them by the backyard mekanik crowd (who probably isn't even wearing safety glasses much less a face shield...)