They can change the way copyright law applies to a work
Whenever you start using a new app or platform—i.e., Instagram, Twitter, or BuzzFeed—you must accept the terms of its usage agreement. That agreement might come in the form of you clicking an "Accept" button or you simply using the app or platform.
As a legal agreement, a license is one way to change how copyright applies within that app or platform. Minimally it will probably permit the owner to make your work public (that does automatically require the creation of a copy) without worrying about you, the author, suing them.
That agreement may supercede copyright law, either giving you more rights or fewer to reuse content. Consider the difference between some rights reserved and (the traditional) all rights reserved.
Creative Commons Licenses
"Some rights reserved" licenses
The Creative Commons organization has created a successful range of some-rights-reserved CC licenses that clearly explain what users can do with works that adopt them. Its website also provides CC search as a way to find CC-licensed images.
Find musicJamendo is a searchable music platform that uses CC licenses.
Find imagesGoogle Images (Go to Settings > Advanced Search > usage rights) and platforms like Flickr provide ways to find images that have adopted Creative Commons licenses.
UseCC licenses Owners can adopt one for their own works. (The Copyright Desk uses one.)
Other "Some Rights Reserved" Licenses
Common licenses that limit the applicability of copyright law
Increasingly creative platforms permit (or require) users to license content with varying degrees of rights reserved.
Instagram "Permissions You Give to Us": "...You grant us a license to use it." "You can end this license anytime by deleting your content or account." (Accessed 31 Aug. 2020.)
Facebook "The permissions you give us": "...You grant us a...license to host, use, distribute, modify, run, copy, publicly perform or display, translate, and create derivative works of your content..." "This license will end when your content is deleted from our systems." (Accessed 31 Aug. 2020).
Flickr "...[You] grant a...right to use the User Content...to display the User Content on the Services." (Accessed 31 Aug. 2020).
“No Rights Reserved” Works
Non-application of copyright law
Works that are in the Public Domain are available for anyone to use without concerns for copyright infringement. Read more about Public Domain.
It is still necessary to cite the author when doing scholarly work. Read more about Plagiarism.
APA (7th ed.)
The Copyright Desk. (YYYY, Month DD). [Page Name]. Arcadia University. Retrieved Month DD, YYYY, from [URL] Example: The Copyright Desk. (2020). Public Domain. Arcadia University. Retrieved September 1, 2020, from https://www.arcadia.edu/landman-library/services/copyright/public-domain