I touched on the why's in the blog post:

TypeScript is also a much stricter language than JavaScript and lets you spot mistakes that could otherwise go unnoticed. It is more convenient to use and allows for quicker error debugging. For developers, very little will change as the syntax is almost the same as JavaScript (Typescript is a superset of JavaScript). It enables JavaScript developers to use highly-productive development tools and practices, like static checking and code refactoring, when developing applications. TypeScript makes perfect sense for growing projects that need to scale and have a variety of contributing developers.