Collections can be used to only show certain posts on your board and not every single post. For example, you only want to show #events. When a post is tagged as #event it will show on that particular Sceen Id. If it is untagged it will be removed. Another example is if you create a Screen Id that is showing the #lobby collection and another one that is showing the #break-room collection. You want your Welcome Company X to just show in the lobby so you can tag it #lobby. You want to add another post for July Happy Birthdays and you want them to be visible both in the lobbby and break room so you would tag that post into both collections.
Another way collections can be used if you are using only digital signage and not web/mobile access is if you want to save all content on your board without having to delete it or assign an expiry so you can use later on. For example, you want to use the same Happy Thanksgiving post next year. You can setup your screens with a collection such as # Live TV Feed and then tag posts into that when you want them to go live and then untag them when you want them taken off the screen. You will then have them visible on your board for the future.