General collections of plants are often made for the purpose of enriching the herbarium, i.e., to produce a collection that mirrors the full diversity of plant life in nature. General collecting to build an herbarium collection is much like librarians seeking all sorts of different kinds of books to place on their shelves. Some herbarium specimens, called vouchers, play an additional specialized purpose. Let's say someone wanted to study leaf development in Chamaesyce maculata. The genus Chamaesyce is large and contains many species that are sometimes difficult to distinguish from each other.
Mistakes do happen.
But if a voucher specimen exists in an herbarium and that voucher is cited in the research publication, the possibility always exists for someone to go back and confirm or disprove the identification. Sometimes vouchers are deposited in great series. For example, any sort of floristic project should have at least one specimen to represent every species included on the checklist or flora. So if it a cold winter's day in Richmond and you want to know what Chamaesyce maculata looks like, or you need some pollen grains or seeds for detailed study, you can bet there won't be any live plants outdoors, but the herbarium collection will provide what you need. The first herbarium collection at the University of Richmond was that of Paul R. Merriman, who assembled a set of vouchers for his Flora of Richmond and Vicinity.