It lets you do the equivalent of "apt install asciidoc; apt remove asciidoc-dblatex" in one command, so avoids downloading the unwanted packages only to remove them.

"The requested action can be overridden for specific packages by appending a plus (+) to the package name to install this package or a minus (-) to remove it"

https://manpages.debian.org/bullseye/apt/apt.8.en.html