The addition isn't going to sway hardcore Pinterest fans, since that service is clearly focused around collecting -- it's really an incidental bonus with Instagram. However, it might scoop up casual users who only visit Pinterest for the occasional inspiration. Why bother if you can get enough ideas from your Instagram feed? And of course, this could be crucial to Instagram's plans for ads and shopping. If you can create your ideal outfit by saving posts into collections, you might be more likely to buy that outfit.