[jrs95]

Yep. And this is important, it allows people to move most JS projects incrementally to TS projects. If they fall far behind the features that many JS projects are using, that won't be true anymore, and those people will start using Flow instead (flow.org)

[WorldMaker]

Similarly it is useful for TS to track the Stage 3 proposals of future features and offer them early, as JS users may pick those up with Babel or other polyfills. Typescript's own transpilation behavior of Stage 3 proposal can sometimes be a useful test for the feature while browsers work on their own implementations.