The variable speed angle grinder is the best way to know what speed your disc is moving at. The reason for this type of tool is so special discs can be used. Not all discs will hold together at 20,000 rpm's. What some people do not realize is the actual speed that the outside of a disc is spinning at. On a small ¼ inch disc, with an rpm of 11,000, the outer edge is moving at nearly 180 mph. Not every job needs that kind of speed nor can all the different types of disc handle it.
Cutting concrete with a diamond disc does not need to be tightly controlled in terms of speed but buffing a car's hood does. A high speed buffer that is variable will leave swirl marks on the hood and possibly burn the paint. This would be disastrous for a detail shop to do. This is just one example of why variable speed angle grinders are very handy to have.
Cordless Angle Drill
If your tool is pneumatic, the speed can be controlled by the air pressure but having the speed control on the tool itself adds to the precision of the control and convenience if more or less speed is needed.
There are two different types of units available that are of variable speed control. There is the slide speed control in which reproducing the same speed is very hard to do but all speeds are obtainable within the tool's range of motion. The preset speed tool allows for consistency when performing the same task repeatedly and the speed is known and needs to be reproduced.
With a variable speed angle grinder, any job from polishing to cutting can be performed with the appropriate speed to do to the job correctly and as safely as possible.