The way I think of this is, if I were running a small team as lean as possible and trying to grow as a business, getting as much out of expensive developer time

The way I think of this is, if I were running a small team as lean as possible and trying to grow as a business, getting as much out of expensive developer time as possible, my answer would be that I get productivity gains sometimes from the work put in to strictly type inputs and outputs to/from functions, whether they be props on react components or whatever. Otherwise, largely a massive time sink that I'd be pissed I'm paying for.