A composer, songwriter, film maker, author or any artist should copyright everything. The reasoning is this. You never know what is going to generate interest and you want to have paperwork for it to move forward. You can’t sell your music without paperwork. It’s like trying to sell a car without a title. No legitimate publisher, music supervisor, music label or anyone else interested in leasing or publishing your music will move forward without copyright.
Copyright protects the author from theft or unauthorized use. Distribution rights are what you sell or lease if you hold the copyright on a piece of material. No copyright, no distribution deal.
A multiplicity of songs can be copyrighted under one fee. If someone shows interest in a particular composition, you can break that song out separately (for an additional fee.)
Copyright exists from the moment your creation is set down in a fixed form; on paper, in a recording. (digital, analog, soundtrack) The problems start when someone comes forward and says “That’s my song” or “I co-wrote that song” and you have to prove ownership. The general rule is: First one to the copyright office wins.
FYI, you give up your right for compensation of anything you post on social media when you sign their agreement. This means that the social media platform you have posted on can use your song, film, photo, etc to promote themselves or lease to a third party without compensating you.
For more information, go to https://copyright.gov/