Do you mean just evaluate the expressions with C-x C-e or C-M-x? If so, you are right that we don't need the SLIME REPL buffer to do so. That is useful for REPL

Do you mean just evaluate the expressions with C-x C-e or C-M-x? If so, you are right that we don't need the SLIME REPL buffer to do so. That is useful for REPL-ing. But we still need SLIME, so that we can send the expressions we want to evaluate to the Common Lisp implementation (SBCL in this case) via SLIME. Since the interactive development environment involves an SBCL process running the Swank TCP server which SLIME running in Emacs talks to, I believe SLIME (or something like it) is still required. For example, when we press C-x C-e, even if we are not using the SLIME REPL buffer, we still need SLIME to send the Lisp expression to Swank which would then evaluate it within the SBCL process and send the result back to SLIME, so that Emacs can show the result in its minibuffer.