One thing I've noticed is depending on how your code is being compiled/transpiled, having smaller defn's over them being nested in a let can lead to better performance since inlining becomes an easier problem.

This is particularly true in clojure, since it's being compiled to java byte-code, and the jvm has a few default settings for how deep it'll look into the call tree for inlining. See https://www.youtube.com/watch?v=0tUrbf6Uzu8 for a talk that mentions this.

You're right, it's not necessary, it's just a preference :)

Although it does 'flatten' the bindings. Defn a, Defn b… is the same as let a = … in let b = … in …