I think that the models 4o, o3, 4.1 , each have their own strengths and weaknesses. Like reasoning, performance, speed, tool usage, friendliness etc. And that for gpt 5 they put in a router that decides which model is best.

I think they increased the major version number because their router outperforms every individual model.

At work, I used a tool that could only call tasks. It would set up a plan, perform searches, read documents, then give advanced answers for my questions. But a problem I had is that it couldn’t give a simple answer, like a summary, it would always spin up new tasks. So I copied over the results to a different tool and continued there. GPT 5 should do this all out of the box.