It's for remarketing which is a part of marketing intended to engage audiences who have already interacted with a brand but didn't convert into a sale at the time they interacted. The idea is that you know they're interested but need to remind them at a later time or they'll forget about it. The main example used for this is when customers hop on a site, add things to their basket, but then leave without purchasing... You know they were interested but something resulted in them backing off at the last minute, a reminder later on results in sales.