When / how to use PKG_CONFIG_DEPENDS?

I'm adding configure script options to a package and I don't really understand how PKG_CONFIG_DEPENDS works.

According to the wiki:
PKG_CONFIG_DEPENDS - specifies which config options influence the build configuration and should trigger a rerun of Build/Configure on change

But isn't this the case for all of the package options? Isn't this the case for all general OpenWrt config options, such as USE_GLIBC vs USE_MUSL and BUILD_NLS? Isn't it the case for most the options found in the dependencies of the package, such as OpenSSL options?
Looking at the other OpenWrt packages as examples, I don't see all of these being added. For example, the Busybox PKG_CONFIG_DEPENDS should be enormous, and yet it's not even present.

So, how to use PKG_CONFIG_DEPENDS? What to add, and what to exclude?

Thank you.