Why is ld50 data so useful in dealing with human health and environmental toxins?

LD50 or TD50: it is the Lethal Dose 50 (to refer to animals), and Toxic Dose 50 (to refer to the human being). To refer to animals is: the dose of a drug, a drug (generally a substance) that kills 50% of a population. To refer to the human being: it is the dose of a drug, a medicine (in general a substance) that produces toxicity to 50% of a population.

Find the LD50, it is very important to determine the dose in which the drug produces toxic effects and thus be able to approach or deduce the effective dose, that in cases of administering a new drug to the human being, it is necessary to find the effective dose 50 (Dose of the drug that produces a therapeutic effect to 50% of the population) in order to determine the Therapeutic Margin or Therapeutic Index.