To give one example, "bootctl" is a small tool that installs the systemd-boot boot loader into the ESP for you. It's a command line tool that synchronously copies a bunch of files into the target mount. We always had the plan to turn that into a D-Bus service, but never actually did it, because doing that is pain: we'd have to turn it into an event loop driven thing, which is just nasty for something so simple that just copies some files.
In a Varlink world, the problem goes away: