'Columns with benefits' and how you can find more than just titles in SharePoint Online
Reusing columns is useful if you want to establish a consistent definition across lists and libraries. You can share the definition across multiple lists and libraries so that you don't have to recreate the column settings each time. A significant additional benefit is that when items are added to a reusable column they discoverable through search (the magic only happens after it has been populated). Reusable columns are called site columns in SharePoint.