My original intention for researching my family tree was to learn how my parents might be closely related, because me and some siblings inherited an extremely rare genetic condition passed down through both of them. It's so rare that a researcher at Harvard (who collected our blood for DNA testing in the 90's) told me that my parents were "probably" closely related.
His comment made my mother upset, insisting to me that she was not closely related to my father at all. Then I tried to calm her down, saying that the scientist said PROBABLY.
Many years later, I did the "Are your parents related?" test at GEDmatch, and it showed that Mom was correct. I don't even have 1 cM (centimorgan) of shared genes passed down from both of them. If that online test had been available in the 90's, I probably wouldn't have bothered with my paper trail genealogy research... which also indicated no shared ancestry between them going back a minimum of 6 generations on all branches.
So I've been autosomal DNA tested, and Y-DNA and mtDNA tested too. It's been helpful a couple times with some old brick walls.
After I started doing it, then the activity grew on me as a hobby for mostly historical reasons.
The Rev. War pension files for my ancestors were particularly interesting! It's nice to "hear them speak" in those old records, instead of being little more than a collection of vital records.
One of my ancestors from the early 1800's was a very old widower who foolishly married a woman in her early 20's! There was a court record for their marriage dissolution that was much thicker than murder trials back then! It was clear that the local court didn't want to divorce them no matter how much the old guy complained that his new wife was sleeping with other men in his bed, allowing her brothers to repeatedly steal from him, etc.!
It looked like they interviewed all of their neighbors about it! One neighbor woman said she overheard, as she walked past their cabin, the new young wife screaming that she would bash in his skull with a log of wood if he dared attempt to throw it into a fire, because it was too hot already!
Their marriage was only dissolved after the young woman's husband was found living in another town. It turned out that she was already married (as one of their neighbors had heard as a rumor), and her husband had left her and her brothers without filing for a divorce. He had been subpoenaed to testify at the trial, and he kept saying how that made him angry and he never wanted to be bothered by the matter ever again!
Edit: He indeed confirmed that they were still legally married, but his attitude was basically, "So what?! What's wrong with you people!"