The biggest selling point of TypeScript is tooling. It provides advanced autocompletion, navigation, and refactoring. Having such tools is almost a requirement for large projects. Without them, the fear of changing the code puts the code base in a semi-read-only state and makes large-scale refactorings very risky and costly.
There are a lot of options available to frontend devs today: ES5, ES6 (Babel), TypeScript, Dart, PureScript, Elm, etc. So why TypeScript?