I buy duplicates of many games (especially favorites), mostly to always have the most current version, because I never want to find myself in a situation where I'm without the ability to play a favored title. For example, of late I've been amassing a number of titles for Switch that I have on PlayStation 4 already, including Crypt of the NecroDancer, Night in the Woods, The Flame in the Flood, and Ittle Dew 2+ (which counts as an updated version more than a port really). Planning on getting the Switch version of Kentucky Route Zero as well. It's just awesome to be able to take those sorts of games anywhere I go and that's a real advantage of the Switch! I'd much rather play those on my lunch or dinner break than 3DS fare.
Do you have any idea how many versions of the original Super Mario Bros. I have? It's a lot!
I most especially like to get multiple versions of rare games. For example, I originally got a fan-dubbed Nintendo 64 cartridge of Wonder Project J2, but more recently have downloaded the computer version because I'm worried about the age of the cartridge and its ability to retain save data going forward at this point.
You get the idea. I'm a pack rat with games. I don't like the prospect of ever losing anything I have, so I typically try to get new versions of stuff I like when they come out.