It's not about readability or "keeping the riff-raff out". Needing to type "return" itself is a minor detail, but explicit returns are a strong sign of having statement-oriented semantics, rather than expression-oriented semantics.

In languages based around statements, things are done for their side-effects. Returning a value is itself a kind of side-effect, hence an explicit return statement.

In languages based around expressions (Lisp, ML & Haskell, APL, etc.), the language itself gently encourages side-effect-free programming. In Scheme, for example, you usually use a begin block when you need to do things for their side-effects. While it doesn't prevent side-effects entirely, it does make them stand out, and adds a subtle pressure against their overuse.