I doubt they're going to tie the favorites lists to personalized boxes or discounts, but I would not be surprised to find out that they're using this to figure out what to put in boxes as a whole. If a large portion of their customers add, say, lip balms, Nuxe products, or stila eye shadow pans but not the all-over shimmer to their favorites lists, maybe they will consider that when putting together future boxes and include more items similar to the items on the favorite lists, whether within the same brands (so more stuff from theBalm or MAKE) or product type (more body oil or natural perfumes), or they can pitch suggestions to brands ("Hey, we have a lot of customers who have added your concealer to their favorites list, but nobody's actually buying it. Maybe we could change that if we sampled it!"). I wish that we could favorite from the main category pages instead of individual product pages because it would make things go more quickly, but I'll take what I can get here.
Of course, this could be nothing more than a wishlist feature that isn't really used for anything more than storing information about what we want to buy, but there's a lot of information that could be mined from this and many ways it could be used, as Amazon has proved. There's an entire movie that wouldn't exist if it wasn't for Amazon suggesting something to some guy based on his past purchase history. He just happens to be one of the most iconic horror directors around, and Amazon decided to suggest a certain book to him, and he ended up making a movie based on that cook.