As you know, the aim of preclinical work (animal pharmacology/toxicology testing) is to build up sufficient data to help in deciding that it is convincingly safe to proceed with human trials of the drug. It is better to be safe than to be sorry (as the English wisdom says).  Clinical trials correspond to the decisive pre-market testing basis for approving a drug. During these trials, this untried novel compound is given to humans and is assessed for its safety & efficiency in treating, preventing, or diagnosing a specific disease or condition. The results of this testing will comprise the single most important factor in the approval or disapproval of a new drug.

Although the purpose of clinical trials is to obtain safety & efficiency data, the prime consideration in these studies is the safety of those involved in the trials so as they are not exposed to unnecessary risks.