Builds a contact list of everyone that watches the videos and engages (Likes, Playlists, Subscribes). Allows you to follow up and promote new material to pre-existing listeners. This works great for Pre-orders, Pre-saves, and Premiers that can set your future projects up for success before they've even been released. It also works for selling merchandise and concert tickets.
Remarketing decreases your advertising budget because you can get better results from reaching less people which means spending fewer dollars.