As you can see, there are a number of reasons that you may not know what reamers are or when they should be used. In most home-based metalworking shops, they will only be used in one of two situations. The first is when the inner diameter is small enough that boring the hole with a mini-lathe is too difficult. The second is that the hole needs to be created in an awkward place and your metal hole saw is going to be too large to be able to do it. While you may only use this handy tool every now and then, it's unique abilities make it a worthwhile tool to invest in.