Although researchers prefer to publish studies with findings backed up by quantitative data, it is not always possible. The physical sciences, such as physics and chemistry, study subjects that can be objectively measured and translated into numbers. They can say with certainty that when you introduce X amount of chemical A into Y amount of chemical B, Z amount of chemical C will form. However, when researchers are studying social structures and more intangible subjects, such as a society's ethics and mores, they cannot collect quantitative data that has the same relevance as in the physical studies.
When physical scientists conduct an experiment, they use their ability to collect quantitative data about a predictable phenomenon either to prove a hypothesis or disprove it. The data qualitative researchers collect are observations and interviews to understand the subject they are studying better. They do not limit the scope to hypothesis testing because to do so would be to ignore many other factors that could help a researcher--and anyone who reads what the researcher publishes--develop a deeper understanding of the subject.
Researchers who use quantitative methods follow a strict procedure to ensure the integrity of an experiment. By contrast, qualitative research is open-ended rather than regimented. Whereas physicist cannot change their procedures until an experiment is finished, qualitative researchers need to change the questions they ask in an interview depending on what a subject is telling them. Because the goal is understanding, rather than proving or disproving, if an interview subject tells the researcher something about the social structure the researcher previously didn't know, he can take the opportunity to explore previously unknown subtopics.
Ethics and conceptions of the world vary from culture to culture. To understand the way a social structure or culture works, the qualitative researcher has to understand the world as the subjects see it. This requires the researcher to put aside her own cultural assumptions to accept the way the culture she is studying works. If she doesn't, then her understanding of the social structure will be clouded by her own preconceptions. Cultural relativity does not mean that a researcher has to internalize and embrace practices or beliefs of the subject society that she would consider evil or repugnant; she just has to be able to set aside her judgments to establish an understanding of the belief's place in the functioning of the subject social structure.