Can I use tapioca pearls for thickening?
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.