In general (at least from when I last kept up), new parts are introduced when LEGO introduces a “theme” of several sets with a similar aesthetic or subject matter at a time. The vast majority of parts appear in multiple different sets, though there can be a few more rare and unique parts. I’d assume that in general once LEGO makes a mold for a part they’d want to re-use it on parts for multiple sets. Rare and unique printed patterns (or if the company is being lazy, stickers) on stand...
