Can I use a rotary tool to sharpen knives and other blades?
First item in sharpening things. When you're sharpening chisels and knives, you don't remove that much metal that you require powered implements. Manual movement on broad, flat stones is sufficient.

Second item in sharpening things. Using a powered grinding implement will destroy any edged tool by heat buildup. A very light touch is needed, once you've exceeded the temper temperature, toss the tool.

A dual sided carborundum water stone will fill the bill for most people. The coarse side takes out the nicks (sign you're maltreating your edged tools) and the fine side puts the honed edge on the implement. Get some polishing compound and a piece of leather to make a strop if you want a really fine edge, it takes the burr off that forms during sharpening.

This is one of those situations where the beauty of having a tool spinning a grinding disk at 10,000 rpm will quickly cause the need for reacquiring everything you attempt to sharpen with it. Any heat buildup takes the temper out of the edge, ruining the tool. Typically, you don't have enough surface area to keep the edge straight either which destroys the straight edge needed on chisels and plane irons. They appreciate the broad, flat surface of any number of sharpening stones.

Same thing with axes, I've seen them destroyed with a common bench grinder, any time the metal turns blue, you've removed the hardness required to keep the edge, at which point, there's no point trying to sharpen it. Best to use a mill file for first sharpening and  one of those Norton Carborundum handled file stones for the sharp edge. These tools work best for axes and soil implements.

Start with the manual methods, learn control and what a properly sharp edge for that tool looks like. Once you've graduated up to where you're sharpening lawn mower blades and other blades (such as brush hog blades) that get driven into media under mechanical power, then start considering using a bench grinder. You need to remove larger amounts of metal to restore the edge, you also need to learn now to take a reasonable amount of material off so you don't overheat the edge and ruin its temper.