I often see debates about which relatives to include in one's research and which to leave out. In general, I think every researcher should do what they find best, and I don't pass judgment on how others pursue their hobby, but my advice is to at least research all marriages of your ancestors and all siblings of your ancestors. Below are my four main reasons for branching out my family tree and researching collateral lines.
Thorough Research is Likely to Result in a More Reliable Conclusion
This is my number one reason for researching collateral lines. In Denmark, patronymic surnames are predominant and researching a man named Hans Hansen is no different from researching a John Smith in records from the US or England. It usually takes more evidence to prove who his parents were. Researching potential siblings, aunts, uncles, etc. is a must in these cases if you want to avoid climbing someone else's family tree.
Source About My Ancestor's Siblings Can Help Tell My Ancestor's History
I feel that I know very little about my great-grandfather Ludvig Hansen's siblings. My mother has told me that her parents did not have any contact with his siblings. So far my research has shown that some of the siblings emigrated, which must have made it difficult to stay in touch, but others lived in the same town, and they still didn't stay in touch! Maybe Ludvig's problem with alcohol and his inability to keep a job for very long was a part of the issue? It would be neat if some of the descendants of his siblings had letters, which indicate why they didn't stay in touch. I wouldn't be surprised if a sibling had written to another sibling about the most recent failures of poor Ludvig.
Distant Relatives Might be Able to Help Identify Persons in Old Photographs
My family has a lot of photographs, which my grandmother Jenny Kristine Juhl left behind. Unfortunately, many of the faces in these photographs are unfamiliar to us. Some of the photos possibly show her aunts, uncles, and cousins on both her paternal and maternal sides. One of my research goals is to identify descendants of Jenny's grandparents because maybe some of them can help me identify our common relatives in these old photos. The photograph at the top of this post shows Jenny's parents Niels Hansen Sørensen Juhl and Gunder Margrethe Elbæk Kristensen.
A Distant Relative Might Also be a Genealogist
Most families probably have someone who is interested in family history. By researching for living descendants of your great-grandparents you might encounter a distant relative with the same passion for genealogy as you. Exchanging research notes with them and maybe even getting together to help each other and share your journey can be rewarding. Listing information about distant relatives on your website (only deceased relatives, of course) is known as cousin bait because one of your distant cousins might see it and contact you to share some information about your family history.
Do you have other reasons for branching out? Share your tips in the comments below!