What is the difference between IRequiresSessionState and IReadOnlySessionState beside the inability of the second to save changes to the session variables?

Both provide me the ability to access session variables in my HttpHandler. But why would I prefer IReadOnlySessionState? It just restricts me from saving the session for the next request.
Or does it gives me an performance advantage over IRequiresSessionState?

When would I prefer to use IReadOnlySessionState over IRequiresSessionState?

c# asp.net session-state httphandler
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