Why do react developers like having a separate types folder/file? (genuine Q)

Not a react developer, but I also like having my types separate from my logic.

It feels cleaner to me.

Real answer: it's a design pattern. Separation of concerns. Back in the days of classic PHP and ASP, logic inside/ alongside the UI code turned into the ugliest code of all time. It was degrading to maintain. Inhumane.

REAL answer: do you mix your steak and beer together when you have dinner? Sure, they end up in the same place eventually, but you wouldn't want to eat beer-soaked steak or drink steaky beer.