If I want to play it with a different group, one the people who have it are not part of, then I consider buying it.
I've also bought some games that I just had to have because they were just that awesome. And then never played, because other people already have a copy, and sometimes they have more content or whatever. Terraforming Mars, for example, I ended up only playing it a handful of times solo, and then the app came out, and I haven't opened my copy since. I try to avoid doing this now, because it really doesn't work out that well.
Although I am considering buying Brass: Birmingham so I can decide when I want it to be available in a meetup. Also Spirit Island, but that's because I think I can get my kids to play with me.
Funny you should mention TFM. That's exactly one of those I'm considering duplicating but both groups I play with already have it...
Spirit Island I used to own but it turned out it's a bit too heavy and long for my GF so it never got played both my groups had another copy and one of them started getting all the expansions. So ultimately I sold it (to another person in that same group ;-) )