I got GTA V with my PS4 and just started replaying again. I'm surprised at the amount of little touches they added from the PS3 version. I just transferred my GTA Online character over to the PS4 and the graphical upgrade is surprising. I'm also glad I got a chance to redo my characters appearance. For whatever reason my PS3 character looked like someone had used her as a punching bag. My point being that I hadn't expected to want to replay the game again but I'm having a blast.
I played it on my old shitty 360 the first time, so having it on my ps4 is fantastic. The small details are amazing, and you're right, playing a game I already played for hours again seems odd but it is amazing.