I made a whole thread on double/triple dipping but here's a few of my personal reasons:
Battlefield 1/Titanfall 2. I originally purchased these games the month they launch for Xbox One. While I enjoyed them initially, I ended up dropping them. I repurchased both on PS4 for significantly lower price on sale (which Battlefield 1 having all of the DLC) because I had purchased a PS4 much later than the original purchase of those games and I ended up having a good handful of friends that had the game on PS4, and didn't even own a Xbox One. So rebuking them gave new life for the game for me.
Sonic Mania. When it releases on Tuesday, I have every intention to rebuy this game. I originally bought it on the Switch, and as you know it was a digital release only. I'll buy the $5 DLC on Switch so it can be portable but I'm definitely buying it again on PS4 or Xbox One so I can have it physical. And I'm slightly tempted to get it digitally again on Xbox One, because of Xbox One's great BC feature, I can have Sonic 1-3&K, Sonic Adventure 1 & 2, Sonix 4 and Mania ALL on one console, that's pretty sweet no matter how you look at it.
Switch's portability, there's several games I plan on buying again so I can have them portable on the the Switch that I previously purchased either on PC, Xbox One or PS4.
Having the game on a modern platform. It's not fun dragging the Wii U to just play one or two games. Same goes for my old Xbox 360 games that's not BC with Xbox One. Yeah I can play MVC3 on my Xbox 360, but that will cause me to hook it up all over again when it's infinitely easier to just buy the new digital version on PS4 with a much better populated online service.
I waited too long between re-releases. This is totally on me. There's a few games where I purchased them, but waited to long to play them and low and behold, a new port got released on better hardware. Again for the sake of convenience (in this case Rayman Legends from Wii U to Switch) I repurchased the game.
I just want a better version of the game. This was true for Pokken Tournament and Bioshock Collection. Pokken added a ton of new characters and DLC which made the original indefinitely inferior. Plus again, online population. While I played through all of the BioShock games extensively, I repurchased them via the Bioshock Collection and replayed them all. I played the original BioShock on my PC and it ran at like 15 FPS, so actually playing the game on a good framerate and non potato graphics pretty much made it a fresh experience. And replaying 2 and Infinite with better graphics and framerate plus the DLC I missed was great too.
I realized far too late that PC gaming isn't for me. So games like SkullGirls and Duke Nukem 3D I have repurchased on PS4 because I just couldn't get into those games on PC. I can't remember the last time I opened Steam and I have a non gaming PC. So there's a good handful of games I repurchased on console that I have on Steam because I would get way more use out of them on console than on PC. Plus it's annoying plugging up the laptop to the TV, awkwardly using the trackpads and looking upward, or having games that straight up won't send a signal to the TV, dealing with corded controllers, it's just a huge unnecessary hassle.
Rebuying a physical game, digitally. I recently did this with Puyo Puyo Tetris as I found myself playing that game in short bursts and found it bbn pretty stupid to change the card just to play for 10 minutes and switch it out with another game.
But probably the dumbest one of all, is when I double dipped in one of the Kingdom Hearts collection games because I accidentally repurchased it because I thought I didn't have it.