To me moving beyond senior means you enable the entire team to contribute together accomplish Big Memorable Things. This can mean a ton of things, which cannot all be served by a single senior+ engineer:

• mentorship
• seeking out, establishing, and evangelizing best practices, and not just coding best practices: architecture, documentation, testing, ci/cd etc
• high-level architecture knowledge and experience
• evaluating technology choices: tooling, databases, orchestration platforms, etc etc
• assisting management and product with scoping and prioritizing work
• the ability to put your head down and crank out a solution to something in code simply because it needs done and you can do it better and/or faster than others
• laying the framework of a greenfield project, maybe sketching out the codebase or POC for juniors to take and run with
• …and so on and so forth.

A single person may be able to contribute all these things to a team over a time frame of multiple years, but in a 3 or 6 month time frame, most mortal engineers could only contribute two or three.