Many people and businesses want to experiment with different models, but they don't want to sign up for a dozen different services. Businesses can make it difficult to approve new vendors. If your company is looking at 5 different vendors for tokens and teams can't agree to switch together, OpenRouter comes along with a unified interface and a single billing point.
They also become the point to add value-add services on top in a portable way. They already offer some things like automatic JSON repair, but I can see them adding functionality like leak detection tools, monitoring, alerts, and other patterns that a company can set up once and use with all of the models theirs teams need.
What I'm not so sure about is their moat. They have the brand recognition, but it seems rather easy for someone else to build what they've built. I'm a little confused about why Stripe didn't just build the same thing internally. Acquiring this company gives them an instant boost of 10 million customers for their AI business, which might be key to some financial goal they've got.