Why are scientists concerned that currently unknown medicinal plants may never be discovered?

Because they could bring many benefits to medicine and provide relief to many sick people.

Explanation:

Medicinal plants have been used for a long time by our ancestors and are known to have an important role in the cure and treatment of some diseases. In some communities, these plants symbolize the only way of treating certain pathologies. It is estimated that approximately 80% of the planet's population has already used some vegetable to relieve symptoms of some disease.

However, there are medicinal plants that are unknown to science, which concerns not only scientists, but also health professionals in general. This is because these plants would provide the advance of medical sciences in a cheap and fast way, besides being able to help many sick people. However, none of this will happen if these plants remain unknown.

the answer is true when observing or determining population size, a direct observation is use, in which each organism is counted in a population or in an area. one benefit of this measure is that you get the exact population count but it will take a long time to  determine  the  size  of a  population.