Question

why are biological dichotomous keys considered to be useful tools rather than taxonomaically significant guides to classifying organisms

Answer 1

In biology, a single-access key (also called "sequential key", "analytical key", or "pathway key") is a key where the sequence and structure of identification steps is fixed by the author of the key. At each point in the decision process, multiple alternatives are offered, each leading to a result or a further choice. The alternatives are commonly called "leads", the set of leads at a given point a "couplet". If the entire key consists of exactly two choices at each branching point, the key is called dichotomous, otherwise it is described as polychromous (or, in false analogy, "polychotomous"). The majority of single-access keys are dichotomous.

It basically helps them define an animal or plant using specific defining characteristics.