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.
I don't think I'm quite there yet. Are you saying having to type return vs. it being implicit encourages side-effects in programming? How does this relate to the begin block?
Mostly, yes. "return" isn't the cause, though. Just a sign that the language's primary concern is the side-effect of running statements, not their values (in which case, having a result value would be expected, not a special case). A sign of different priorities in the language design: functional-by-default or imperative-by-default.
The begin block is how one indicates "do this, ignore its value (just do it for its side-effect), then do that and return its value" in Scheme. Some forms have implicit begins, however.
I may just be repeating myself, but I hope phrasing it slightly differently helped. What made it clear to me was learning Scheme and (especially) OCaml, then going back to Python and programming in a functional/expression-oriented style, seeing the subtle ways in which the design of the language resists it. Lua is more friendly to functional programming than Python (it has tail-call optimization, for starters), but those explicit returns still show it isn't a perfect fit.
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.