Once you return a spirit to its constellation, you can meet them in person at the beginning of a level's hub. You can offer them light tributes and you unlock new expressions, new outfits, and new spells for your sky child. This is also one of the few ways to earn hearts, so you will need lots of light tributes. Just keep going near red candles, burning the darkness, and rescuing spirits and you should be able to get a lot of light tributes.