Many people think of doing "genealogy" as finding those direct line ancestors, our grandparents, great-grandparents and beyond. However, what about finding the living descendants of our ancestors? Are they not also family? They are our collateral line cousins.
Why should we do descendancy research? Probably the biggest reason in my mind is the fact that they may also be holding on to information about my ancestors that I may need. It is amazing how many of my collateral line "cousins" have photos of my ancestors. My ancestors 2-3 generations back also shared photos, letters, and such amongst their siblings or family members. I may have information that a collateral line cousin could use. Collaboration with these cousins that we have never met becomes quite important. The large databases like FamilySearch, Ancestry, MyHeritage, etc, give you the ability to contact others who maintain trees and are providing information and media to their trees. Take the next step and contact them.
A cousin that I have never met, recently contacted me on our common 2nd great-grandfather. He had found a family bible page with the names of this 2nd great-grandfather's siblings on it. Suddenly this dead-end-line opened up in both directions by finding this information.