When a sociologist uses primary data, she has maximum control over how data is collected, who it is collected from, who it is collected by and for what purposes it is used in her study. Secondary data collected by another sociologist at another time remains somewhat disconnected from the sociologist who is trying to use it for her own study; as a result it will not be tailored to the question she is attempting to answer.
Most sociological studies set out to answer a question or at least explore possibilities or a hypothesis. When a sociologist does her own study, she can be aware of the biases that she brings to the research and the writing of that study. On the other hand, the sociologist will have no way of telling where the bias of another sociologist lies in his own paper. What she takes for factual data in the paper of another sociologist may simply be opinion.
Sociologists attempt to determine the "representativeness" of every study they conduct themselves and the studies of others that they use to back their claims. Representativeness measures how accurately a study truly represents the person, people, community or group it claims to represent, and how accurate the data is to reality. When a sociologist is using primary data, she is at the scene where data is collected and is observing the data in real time. The sociologist can therefore be reasonably certain that the data she records is reflecting reality.
Primary data stands as a useful comparison to secondary data. By finding primary data, a sociologist can see how her data measures up to a study done by another sociologist on the same topic. If the data does not match, her new findings must be justified and may result in new information that will discredit a former belief.
When reporting on the level of an individual or a small, intimate group in a sociological study, it is impossible to get the appropriate depth necessary if only using secondary data. Recording words, thoughts, feelings and observations directly from primary data is a type of depth that truly solidifies a study and makes it relatable to general readers.