A few months ago a cousin who was previously unknown to me came across this blog while she was Googling the name of her grandfather (an artist) and she contacted me. We exchanged a few e-mails and I was finally able to make clear to her what the relationship was. She is my third cousin once removed. Her great-grandmother Elka is the sister of my great-great-grandfather Abram. Not close cousins but not too distant either. While I was asking her questions about her family she asked why I was researching the family of her great-grandfather Benjamin, Elka's husband.
To people who are not obsessed with their family histories this may seem strange...why research people who are not blood relatives? To people who are obsessed with their family histories, the answer is obvious...a cousin is a cousin is a cousin. All information I can find out about any member of my family tree extends the branches of the tree and there is also the potential for that information to come back full circle to my blood line. It was not uncommon to see cousins marrying cousins. Or maybe two brothers could marry two sisters and suddenly you've just discovered a sibling you didn't know about.
Another reason researching all parts of the family tree is so I can have information on hand in instances like this when someone contacts me. It's a way to break the ice but can also be used to subtly barter for information they might have such as old photos of people they can't identify (they might be your family members at that wedding reception table). This process can get some people drawn in to the research while others just fade away.
Mostly it's probably just the thrill of the hunt, being satisfied with myself that I solved a mystery or made someone happy to be holding their great-grandfather's ship manifest. It's just fun and I'll keep going even if they don't understand it.