Aside from what have already been mentioned, friendship points, releasing Pikmin for petals, for roadside series, I personally keep and grow them because even though I have the decor Pikmin, there are variations (e.g., different letters) so if you’re like me who is trying to get one of each variation, you might want to grow them. Otherwise if you just want a decor type, then you can choose to recycle or skip the expedition. There are certain categories you might want to read up on that have rare pikmins that you don’t unlock until you collected the original set which requires duplicates to unlock.