In August 2019, reddit user zzstormriderzz asked this question.
The latter is useful at all times. In my opinion, it speeds up development by essentially telling you what code you have to write next. It’s not a huge improvement, but it keeps you on track and wards off context shifting, such as a coworker interrupting you to ask a question. Where was I? TypeScript tells you that your function needs to return the other of its two cases. Cool.