As a project gets larger (more code or more devs), each dev spends more and more time figuring out what the correct types need to be when using various internal APIs. Refactors get harder because it becomes harder to know whether you've correctly found and changed each place that was needed.
And although all projects have this to a certain degree, one typical goal of a commercial project is a higher quality bar for software while still optimizing amount of effort to get that quality. (And commercial projects are often larger too.)
TypeScript helps with these issues (and others). In my opinion, it increases developer productivity for all but the smallest JavaScript projects. It's a combination of both the static typing and the tools available that help the dev immediately, as opposed to waiting for a compile or test run.
I'm actually passionate enough about the topic that I have an old blog article about it: https://medium.com/hackernoon/3-reasons-why-we-use-typescrip...