Jenkins does offer a web UI to manage plugin installation from a live master instance, but in many deployments administrator would like to control installed plugins by version and by tooling, before jenkins master starts.

• docker official image provides an install-plugins.sh script which evolved far beyond its initial "help script" scope.
• configuration-as-code implemented plugin installation hack based on PluginSite.Plugin#deploy. (Removed as of v1.8 of JCasC plugin)
• open-source update center does expose plugins-version.json on casc request so one can manage versions for all hosted plugins, not just latest
• custom war packager does support plugin installation on his own
• jenkins evergreen does manage plugin installation based on updates from metadata server, using his own logic implemented by nodejs evergreen client.
• jenkinsfile-runner does manage plugin installation implemented in Go
• coreOS provides a groovy script to handle required plugins
• cloudbees DEV@Cloud uses a set of Chef Ruby recipes for the same purpose
• etc

Everybody is re-inventing the wheel, partially implementing the "details" of plugin management (signed metadata, artifacts checksums, plugins detached from core, ...). It becomes obvious Jenkins should provide adequate tooling for plugin installation outside a live Jenkins instance.