Herbarium specimens consist of dried and pressed plants (or lichens and mosses) that are meticulously identified and labeled with location information for where they were collected. I have used them to help teach the Plant Systematics lab class at Michigan State University, and am making my own collection of pressed plants from my field research this past summer, which greatly helped me in properly identifying the species I saw in the field. Many closely-related species have microscopic details that tell them apart from each other, and you cannot accurately identify them outside while you are in the field. Instead, you need to bring specimens back to the lab to look at the morphology of their seeds, hairs, or length of their floral parts (as a few examples) to tell species apart. Some of the useful web resources and hard copies of flora that I use to learn county presence of certain species assemble information from pressed plant specimens deposited into herbariums. (I like using Calflora for California research and Michigan Flora for my Michigan research). This highlights how important botanical collections can be, even for people who aren't studying taxonomy questions but are doing ecological research, where it can be important to document species richness, abundance and diversity for various plant communities. I am particularly interested in exotic, invasive species, so herbarium specimens can act as records of the first documented presence of an exotic species in an area.
The article also highlights how important botanical collections are for studying climate change and how species distributions are changing over time. Considering that pressing plants was a common activity of botanists in the late 1800s, hebariums do have, in some cases, an adequate number of specimens for certain locations from over 100 years ago that can be used to compare to specimens that have been documented in the same locations now and in the decades in between. Sometimes DNA or RNA can be extracted from herbarium specimens for genetic studies, and evidence of disease that causes discoloration or changes in morphology can provide valuable clues on the spread of disease through natural plant populations.
I do hope that trends will change and the specimen collections of natural history museums and botanical gardens, as well as universities and other institutions, will start to receive more funding to preserve their collections and the necessary curator and research positions that are needed to allow scientists to make important discoveries about our natural world.
*note: the article is not restricted to herbarium specimens of plants, but this is what I chose to focus on as this is my area of expertise. Many zoological collections are also experiencing the same threats as well.