As a Racket dabbler, I’d say there are a couple things:

- performant implementations (SBCL in particular is a standout). I mean, Racket’s pretty good, and will typically blow the doors off Python, but SBCL is in a different league.

- multiple implementations conforming more or less to a standard. This can also be a distraction, but is on balance a nice thing to have, particularly if you wanted, for example, an implementation that ran on the JVM (ABCL).

- different set of libraries. Racket’s library situation is pretty good, but sometimes CL will have something that Racket doesn’t (and sometimes the other way around)