Remember that we are building on a modern platform for which browsers still lack support. Therefore, directly referencing script files in HTML is out of the question (while common, it's a dated approach that we should avoid anyway). Browsers do not understand TypeScript; this implies that there has to be a process that converts code written in TypeScript into standard JavaScript (ES5). Hence, having a build set up for any Angular app becomes imperative. And thanks to the growing popularity of Angular, we are never short of options.
If you are a frontend developer working on the web stack, you cannot avoid Node.js. This is the most widely used platform for web/JavaScript development. So, no prizes for guessing that most of the Angular build solutions out there are supported by Node. Packages such as Grunt, Gulp, JSPM, and webpack are the most common building blocks for any build system.