Samsung announced the Bixby assistant several weeks ahead of the Galaxy S8 launch. We were told at the time this feature would let you control apps by voice and get contextual information. It doesn't do any of that at launch, though. Naturally, people want to remap the Bixby button, and that's still possible despite Samsung's attempts to block it. However, the remapping apps are pretty hacky.
Bixby remapping apps are sprouting from the Play Store like weeds in an untended flowerbed, and they all seem to have positive reviews. People are so anxious to do something useful with that button they'll put up with a lot of weirdness. Samsung's first OTA update to the GS8 made Bixby remapping more difficult. Now, these apps have to run a listener service to trigger actions.
I've tested three of them on the GS8, and each has its own... quirks. Bixby Button Remapper and Custom Bixby Button both work as advertised, but the Bixby Home panel still pops up when you hit the button. It just closes after a fraction of a second, then your chosen action is triggered. It's not pretty, but it works. bxActions is a little different. It doesn't open Bixby Home before launching your action, but it does send you back to the home screen if you're in an app.
There's some concern these apps will cause lag on the other hardware buttons, so maybe it's safer to just avoid them. People are willing to put up with the foibles because Bixby just isn't very compelling. I understand that Samsung never intended Bixby button remapping to be a thing, but making it more difficult is not a very consumer-friendly thing to do. So, these hacks are what we're left with.