SystemJS - TypeScript loader use cases

I have a question regarding following TypeScript plugin for SystemJS :

https://github.com/frankwallis/plugin-typescript/

Here is its description

A plugin for SystemJS which enables you to System.import TypeScript files directly. The files are compiled in the browser and compilation errors written to the console.

I wonder what would be the use cases of such plugin.

Why would developers import directly ts files and compile them in the browser instead of compiling them during development and import js files ?
Won't it reduce performance and load time to do it in browser ?
Is it supposed to be used only in development environment ?

Solution

plugin-typescript author here. In-browser compilation is strictly a development tool, in production you would use systemjs-builder (in combination with plugin-typescript) to create a single file containing all of the transpiled javascript.

Since the plugin was originally developed, a number of new workflows have become available when using typescript & systemjs (typescript single-file transpilation, vscode, systemjs hot-reloading, typescript system.register output, to name a few...) - Which one is right for you will depend on the size of your application, the platform/server you are using, and your own personal preferences.