Adding fics to AO3 collections as a reader
Depending on what other sites you read on, you might think of a Collection as a kind of bookshelf for stories you like. And it can be that, but that's not the only thing collections were intended for.
According to the FAQ, "A collection is a group of works and/or bookmarks compiled together under one theme." They can be used for a fandom fest or challenge. They can be used by an author to organize their works. And they can also be used by readers to organize the stories they've read.
The important thing to keep in mind when adding something to a collection is: are you adding the fic or are you adding a bookmark of the fic?
If you, as a reader, try to add an author's fic to your collection, that author will get an email asking them if they want to let you do that. A lot of authors will say no. This is because the owner of the collection can hide all of the works in a collection, making them invisible to anyone else on the Archive. This feature exists to support fandom exchanges, challenges, and fests where all of the works get revealed on the same day.
If you, as a reader, want to create a collection then I recommend bookmarking the fic and checking off the box in the bookmark form that says "Add to collection." This will add just your bookmark to your collection, not the fic itself. You will not need the author's permission, and nothing you do will affect the fic itself or hide it from the rest of the Archive.
Collections of bookmarks can be a great way to organize your favourite fics, and you can add a bookmark to more than one collection at a time. You have a fic that is about your favourite character AND it's fluff AND it's canon compliant? You can add it to a Character Collection, a Fluff Collection, and a Canon Compliant collection and then no matter which of those moods you're in, you'll still have it right there waiting for you to read it.
You need to create your collection first, and it will take a couple hours for the collection to appear for you to add bookmarks to it, but once you've got your collections set up, you can just keep adding bookmarks to them pretty much forever.
tl;dr collect bookmarks, not fics and organize your bookmarks in a whole new way
addendum: if you are an author and you have allowed your work to be added to a collection, you can remove it at any time
