What was Durkheim's study of suicide and why does it matter?

It is first necessary to understand why Durkheim carried out this study in the first place and what he was trying to achieve by doing so. As you may remember, Durkheim was a structuralist, and thus believed in the importance of wider social factors and groups in explaining behaviour, as opposed to others who emphasise agency or the importance of the individual. If Durkheim could show that suicide (which at the time was viewed as a very personal, individually motivated action) was actually related and caused by social factors, he would provide strong support for the view that structure/society is indeed what causes behaviour rather than agency. 

Indeed, this is what Durkheim found: it was shown that suicide could be predicted by the level of a) integration and b) regulation in any given society. Note that these are social factors, wider than the individual. Durkheim had therefore taken something that seemed so personal, and related it to wider society, in a way that had not really been done before. In this sense, the study stands as strong support for the argument of structure over agency, or the importance of groups over the individual.