Biostatistics are vital to the field of public health because they provide public health professionals with the basic sorts of information they need in order to do their jobs.
Public health can be defined as the study of how to protect and improve the health of the general public. Public health officials try to do such things as educating the public about dangers to their health.
Biostatistics in this context are statistics gathered about health trends. Such statistics can tell us, for example, that smokers have a higher rate of lung cancer than nonsmokers.
Public health officials can take biostatistical data and use it to determine what sorts of efforts they can make to improve the health of their constituencies. Therefore, anyone who wanted to make a career in a policy-making level of public health would need to have an understanding of biostatistics.