Quantitative research is often given more credibility because it is perceived to be more "scientific" - which can lead to researchers (or those commissioning the research) giving preference to methodologies that utilise quantifiable/measurable data. This can have serious repercussions in certain fields of enquiry (eg: education) where certain "outcomes" (such as numerical test results) might be considered to be more easily measured and given preference despite them not being the most appropriate indicators of desired outcomes.
Quantitative research employs the scientific method - which may involve setting up experiments; manipulating variables; observing and measuring effects; collecting and analysing data. Because there may be ethical implications in carrying out certain kinds of experiments in the social sciences, the methodological approach employed will likely depend upon observation of differing but naturally occurring variables and their effects.