What is a scenario where [GLOBAL].build_ignore would be used instead of [GLOBAL].pants_ignore?

there might be a few, but at least we use it to prevent creating build targets:

build_ignore = [
    # This is here to avoid tailoring requirements.txt there
    "custom-tools/.BUILD",
]

you may still store Pants related files in that directory, e.g.

.lock

files which you do want Pants to interact with