A chemical in the bark of Canada yew (Taxus canadensis), for example, has been used in various cancer treatments.
American shoregrass (Littorella uniflora) is only found in very nutrient poor lakes in northern Wisconsin. As nutrients run off the uplands into the lake, this and other similar species will disappear.
The lacy flowers of eastern prairie fringed orchid (Platanthera leucophaea) add tremendous beauty to a tallgrass prarie and inspiration to any artist.
Although the more common species like black spruce in the swamps of the north or big bluestem in the prairies of the south often define the community type, there are other examples where individual animals require specific plants to complete some part of their life cycle.
One such case of dwarf huckleberry (Vaccinium caespitosum), which is the obligate host for the larvae of the state endangered northern blue butterfly. There are many similar examples with plants not quite as rare as dwarf huckleberry. All species, from the most common to the rarest, however, have intrinsic value. That is, although we may not be able to clearly point to an animal that depends on it or a drug that can be synthesized from it, all species have value as a piece of a larger puzzle. To only consider the utility of a species, what it does for us, is to overlook all the intangible benefits that species provides. As Aldo Leopold said, "To keep every cog and wheel is the first precaution of intelligent tinkering."
Some species have always been rare. They require a habitat that has always been rare in Wisconsin. There are other species that were more common but have declined recently due to usually human-induced threats such as habitat loss, disease or poaching among other things. When it comes to rare species protection, management will is most likely to be effective, and is perhaps most needed, for species which have recently declined.
Most rare plants are restricted to certain habitats, or natural communities. Understanding the connections between plants and their habitats will help you prioritize your survey efforts once you get to a site, helping you avoid wasting precious time. For example, if you know a plant is usually found on dry prairies, then you may want to focus on steeper, often south-facing slopes. If the plant you are looking for is usually found in rich, mesic forests, you should first look for north-facing slopes with larger trees (a sign that the area hasn't been logged recently).
You can also learn about the natural communities themselves. The Wisconsin DNR has excellent descriptions for each natural community found in Wisconsin, including indicator species, soil type or moisture and location in the state. Learning about the habitats you will be surveying is important preparatory work volunteers should do before each survey to ensure the best chances of relocating the target species.
Remember, it's not important that you relocate the target species, but it is important that you adequately survey the most suitable habitat.