why did lewis and clark collect samples of plant life?

Answer 1

Most likely it was to determine what could grow where and in what type soil as they traversed the country. Some plant life may have been new to them; hence, the collecting.

Answer 2

For the same reason that we collect rock samples on the moon and map Mars so extensively. They were cataloging the land. What kind of soil, what kind of plants grew there, mineral wealth, water access, etc.