Keep the maintainers in mind

Makes packaging for e.g. Debian much easier, and makes the maintainers feel better that all their licensing ducks are in a row. Also makes it easy to make sure incoming commits have proper copyright/license if you use something like https://reuse.software/ (which is basically "enforceable spdx headers, with tooling") because you can have a CI job checking it, etc.

Particularly useful if you e.g. publish a standard with open source parts, you can make sure even the internal repo is properly annotated and won't give you headaches at release time. (From experience here - as part of my job, I do spec editing on a Khronos standard, as well as working on open source software)

I'd also say that having a standard license tag at the top of every file, instead of just a license file for the whole repo, helps avoid "there was no license so I copied the code" thing, which is of course wrong but doesn't mean it's not done.

Also helps me re-use the little "ancilliary" files that often get ignored for copyright/license headers, like scripts, CI configs, etc...