Genealogists are often familiar with the importance of working on not just immediate ancestors, but neighbors, slightly-more-distant relatives, and associates. Information on these individuals can sometimes give either direct or indirect insight into the ancestors in question.
And if your ancestor was involved in any sort of criminal activity, do you know who his (or her) partners-in-crime were? Those associates can be clues as well.
For a longer post, read “Partners in Crime.”