Need to? No, of course not. Im using the Moto Z Force without any issues, but the S8 looks really nice. Also, the camera on the S8 is probably a huge upgrade from my Moto, but I could just pull the trigger on the hasselblad camera mod, or buy an actual camera if I feel the need.
On a sidenote, while I'm not needing an upgrade, I'm very excited to hear about new flagships and see the design choices they're making, as it says a lot about where tech is headed. I'm loving how Samsung and LG are moving towards smaller bezels and I hope other OEMs follow suit.