I've tried making Cacio e Pepe a few times, using a recipe from America's Test Kitchen. In Cacio e Pepe, you boil pasta in a limited amount of water, then mix some of the hot starchy water with grated pecorino Romano cheese. The cheese melts/dissolves in the water to form a cheesy liquid which is poured over the pasta.  I've had good luck doing this with Romano, even using domestic cow's milk Romano. But one time I was short on Romano, and tried using about 1/4th Romano and 3/4ths imported Parmesan. When I added the starchy water, some of the cheese dissolved--the Romano, I assume--but most of it coagulated into a gooey, stretchy mass.  What is the difference between Romano and Parmesan, that would make them behave so differently when mixed with hot, starchy water? Are there other similar cheeses--asiago for example--that would work well in this recipe? 
The problem with using the non-ground pearls is that they don't always fully dissolve. Cook's Thesaurus indicates that they can be pulverized or ground in a grinder or by mortar and pestle and used similarly to the starch. However, there are pearls which are instant and those that are not. With instant tapioca you can substitute 1:1, with regular tapioca pearls, pulverized, it would be 2:1.  As for What are they good for?, they are good as a thickening agent assuming that you are okay with little gelatinous balls in whatever you are making. Rice and tapioca puddings spring to mind; there are uses of it in curries and stews for its thickening prowess; also it can be used as a gluten free thickening agent in place of flour.  If you run out of corn starch or arrow root you can also turn to the pulverized stuff. Same rules apply; slurry 1 Tbsp in cold water per 1 cup liquid to thicken. However, do not bring to a boil or you will end up over-cooking the starch. 