I normally use them when I need a bigger hole in steel. Once I get close to a 1/2" drill size I know the cutting forces are going to get pretty high so I switch to a rotabroach. I also use them when the work piece is large and unwieldly so that clamping it to the drill press table is a hassle. There are times when the work cannot be drilled on the press or mill, like a hole in a car frame or whatever. The rotabroaches work just fine in a hand held drill but lubrication makes a big difference in cutting speed and the results - faster and cleaner.
If you have ever had to use a hole saw to cut something in metal then you know what a hassle that is and what the results are. I used to dread cutting/drilling a large hole in steel but not anymore. If you have ever used a Silver and Deming drill bit in a drill press or hand held drill to cut a big hole then you know how long it takes to work up to your final hole size and the risk you take with your hands and wrists when they catch. No more step-drilling with rotabroaches - just go to your final size and drill it quick.
I could go on and on. These tools are not a panacea for a drilling issues. There are times when a drill is better, like when fit is important or when clearance issues prevent getting a rotabroach in place but for the most part, they are better for drilling thicker materials and they don't leave a big burr for you to clean up. Give them a try.
