essentially I don't understand why variables like RDEPENDS require a package name conditional override such as RDEPENDS_${PN} while other variables, including DEPENDS, do not require this. Isn't putting the package name as a conditional after the variable pointless? I feel like my confusion may stem from some fundamental misunderstanding of the way bitbake works.
When a recipe is built, that single recipe can generate multiple packages. For example, debugging information is in ${PN}-dbg, docs in ${PN}-doc and development headers/files in ${PN}-dev. The "main" files for a recipe would go to ${PN} but many recipes split other pieces into other separate packages by adding entries to PACKAGES (which defaults to the above values).
Since there are multiple output "runtime" packages, runtime variables such as RDEPENDS have to be applied to a specific output package, hence the RDEPENDS:${PN} or for older releases RDEPENDS_${PN} variable name format, otherwise it would be unclear which package they applied to.