We have to separate the license terms from the license itself, and from other notice requirements.
- The license terms are about what the recipient is allowed to do with the covered work.
- The license says “this work is covered by those license terms”.
- There might be other notices. Copyright notices are usually not legally necessary, but provide nice attribution (“I wrote this!”) and also indicate that a valid license was given (“Because I wrote this, I am the copyright holder and am allowed to issue you this license”).
When you are asking about license snippets, you're likely asking about the kind of license headers that are often plastered into each file of a project. These notices usually contain a copyright notice plus the license.
Many permissive licenses (MIT, ISC, BSD-3-clause) are so short that we use the full license terms as the license notice.
Many longer licenses such as the GPL licenses or Apache-2.0 contain an appendix that suggests a license block that you can use in this manner, while keeping the license terms in a separate file. The SPDX license list collects license terms that are Open Source or open-source-like. If you click on the entry for a license, there's a section about the “standard license header” for that license, if it exists.
About copyright notices: There are international agreements such as the Berne Convention and the TRIPS Agreement. Nearly all countries1 have signed one of these agreements. These treaties say that copyright is automatic, without requiring copyright notices. Why then are copyright notices so common?
There are a couple of more banal reasons:
- They provide nice attribution.
- They make it feasible to track when copyright will expire.
- They prevent a defense of innocent infringement (if there's a notice an infringer can't reasonably believe the material was free of copyright)
- Removing or falsifying notices is still pretty illegal.
But more importantly:
- It's just really common in the Free and Open Source software community.
- This community and its licenses are strongly influenced by the United States.
- The US were really late to join Berne, and did previously require copyright notices in order to have copyright protection. This ended as late as March 1, 1989. By that time, there were already lots of licenses, projects, and communities that we'd now consider “open source”. E.g. the MIT license was drafted around 1984 and the first GPL version was published in February 1989.
The appropriate format of copyright notices ultimately depend on national law, though US conventions are common. The GNU maintainer handbook has a section on copyright notices which explains some conventions, though it notably deviates from the format prescribed in US law in some details (such as allowing ranges of years).
In practice, copyright notices are handled fairly flexibly. Sometimes the years are never updated. Sometimes the name in the copyright notice is something like “the FooBar project contributors” which is not a person and unable to actually be a copyright holder. However, I prefer this over listing concrete names (such as the name of the project's original author) to avoid the impression that only the listed persons would be copyright holders – unless there's a CLA/CTA involved, every contributor is the copyright holder for their contributions, regardless of what the notice says.