Some of are fortunate enough that a significant amount of their genealogy can be traced relatively easily without researching all the individuals with whom their ancestors were associated. Some people do leave behind enough detailed records that accurately uncovering their parents, spouse(s), children, and vital events is not overly difficult. Personally it was easy for me to trace back to my great-great-grandparents without doing a whole lot of work.
Then it got more difficult. My relatives were on the frontier in states that did not keep vital records. They moved frequently. Not all of them owned land or left records that detailed their existence. Some left very few records at all.
In those cases searching the individuals with whom they associated became even more important as those individuals who appear on other documents with my ancestor may not have been random people who had absolutely no connection to my ancestor.
- The witness on a deed may have been a brother-in-law, the brother of the man selling the property
- The witness on a will may have been a step-brother of the man who wrote the will.
- The men who serve as bondsmen when the widow is appointed administrator of an estate may have been her brother.
- The man who signed a marriage bond for a bride may have been her uncle or brother of her first husband.
- The man who makes an affidavit in a widow's pension where he states that he knew she was married to the soldier fifty years ago may have actually been her brother, her cousin, or former neighbor "back east."
- The man from whom your ancestor bought their first property after heading west may have been your ancestor's cousin or other relative.
And even if your ancestor did leave behind adequate records that allow you to trace them, learning something about the associates may allow you to paint a more detailed accurate picture of your ancestor's life--and get beyond the vitals, names of parents, and children.
There's other reasons besides these to search associates...the key is to remember that unless your ancestor was a Hoover, they didn't live in a vacuum.