It probably is mostly used for dead works. I feel it's just the simplest way to describe that the user can do whatever they want with it. But the problem is this doesn't seem to actually work, as I said, some jurisdictions don't actually allow one to waive their copyrights or moral rights. If it was able to work another benefit can be for things that are one off projects or small snippets, say you release some code only 40LOC and you want to make it as free as possible. If you were able to place that in the public domain that just simplifies it, whereas licensing it would just feel like overkill (the license would probably take up 1/4 of that file). But as there doesn't seem to be a simple option for doing that, the license is likely best. There also can be a good reason for large projects to be put in the public domain, compared to licensing; when it comes to the ideals of the project and how copyright law can work or even change overtime (e.g. it copyright could be passed onto someone that didn't share the same ideals of the project; although that is where a copyleft license could come in but a copyleft license is then adding restrictions).

If you do put something in the public domain there's nothing stopping you from mentioning the authors or how to contact you. Just someone is free to remove those authors, or even try to claim they authored it themselves. And SQLite doesn't use Unlicense, they have their own waiver and other statement.