Note this part of my statement: "It also prevents you-the-designer from being tempted to add interactivity logic at design time."

In practice, in many industries, prototyping tools are used as the first step in the design process. The constraints built into these prototyping tools force those using them to spend less — often orders of magnitude less — time, and effort, and money(!) prototyping, than they would if they allowed themselves to prototype with production-oriented tooling.

Consider painting. A painter — unless they're working off of a photographic reference — will almost always draw a pencil sketch of the scene they want to capture in paint, before they begin the actual painting. A pencil sketch has no need to consider color (or how to mix to achieve particular color effects), or light and shadow, or dimensionality (how real light reflects off of built-up paint on the canvas), or any of that. They just need to concentrate on proportion, perspective, anatomy, etc. The sketch focuses only on (a subset of) the design of the painting, while inhibiting any of the implementation details specific to the medium of paint, from being worked on. Which means the sketch only takes a few minutes, rather than days.

Once they have this sketch, they can show the sketch to the client who commissioned the painting (or to the master of the studio, if you're painting for gallery sale), and the client/"product owner" can point out places where the sketch does not align to their vision for the painting, which can be used to iterate on the design.

There are many other things to get right once the "real" painting starts, but if you don't get the "bones" of the thing right, the client won't want the painting. The sketch lets you evaluate just the "bones" of the painting before even considering the meat on those bones.

A prototyping tool for programming should be the same: something to let you consider the "bones" of business logic, preconditions/postconditions, etc., without the "meat" of the particular library APIs and data-structure juggling required to glue things together and achieve scalability in a particular language.

The best prototyping tools are ones that pare down your focus to the smallest useful kernel of design, and thereby allow you to iterate in near-realtime. An experienced painter will become skilled enough at making sketches, that they can sit down with a client and sketch in response to the client's words, changing the sketch "interactively" in response to the client.

In fact, some prototyping tools are streamlined enough to allow the client themselves to iterate and ideate on the sketch by themselves; and then only submit it to the productization process once they're happy with it!

Game-development example again: RPG Maker. As far as I can tell, RPG Maker as a software product was never really expected by its vendor to be used in the production of commercial games (although it has been repeatedly marketed that way.) Until very recent releases in the series, it was far too constrained for that — unless you entirely eschewed most of its engine [as most of the RPG Maker "walking simulators" like Yume Nikki do], it gives you a very static set of engines: battles that work exactly one way, menus that work exactly one way, etc. A tool that's actually for building role-playing games as commercial products, would have an almost-monomaniacal focus on letting you customize these systems to make your game distinctive; but RPG Maker is entirely the opposite. Rather, I believe that in its idiomatic use, RPG Maker has always been intended as a tool to allow a client to "sketch out" the narrative(!) "bones" of an RPG. That's why it gives you so many built-in assets to work with, but also why these assets are so generic, and also why ASCII/Enterbrain never created an "asset store", nor documented the asset formats: the assets are meant to act essentially as wireframe components. You're not meant to re-skin an RPG Maker game; you're meant to just use the generic assets to build a generic-looking game, because the look of the game isn't the point, any more than the look of a pencil sketch is the point.