[1] The main goal of a dose-ranging study is to estimate the response vs. dose given, so as to analyze the efficacy and safety of the drug.
In addition, it is easy to recruit and select volunteers among varying age, sex, race etc.
[2] The duration of action should be determined during dose-ranging study, as it will allow definition of the dosage schedule.
Because it is hard to measure reliable pharmacodynamic parameter, it is difficult to determine the duration of action during early clinical trials.
Other parameters instead are suggested as a tentative dosage, such as half-lives in plasma and urine in various test species and human, receptor binding in vitro, or pharmacodynamic data in vivo in animals.