Couchers Frontend Team Lead here. Aapeli is out for the night so I'm popping in to answer from my POV.
I think the main difference is that we're trying to capture the spirit of what CouchSurfing.com used to be: modern, easy to use, welcoming to newbies and centered on genuine social connection. But we also want to go beyond that. Build for today’s world—with better safety tools, better moderation, and more community-driven features that help people find each other easier.
Couchsurfing was initially about free hospitality and cultural exchange but is now largely driven by monetization. They also haven't really provided many new features to users since going for-profit.
BeWelcome is another alternative that came out of the CouchSurfing community years ago. It has a more ideological focus around democratic decision-making and they are not as newbie friendly, have an older UI, and are a bit slower to adopt new tech.
WarmShowers on the other hand is for a completely different crowd: it's for bike tourers that leave at the crack of dawn and arrive at sunset. They need a shower (hence the name), a place to put their bike, and a bed to sleep on. They'll probably be a bit too tired to socialize. That's very different from the traditional couch surfing platforms where socialization is the focus.