> Why would a user want to do plugin managment on the server in their Jenkinsfile? It would be like gitlab ci users upgrading their gitlab CI version via gitlab-ci.yml.

This means that when a developer need a new plugin they need to ask the administrator of the Jenkins server, frequently a central IT organization.

With GitLab all the functionality is always enabled, there are no plugins to install.

I disagree that installing plugins is the same as upgrading GitLab or Jenkins itself. Although of course GitLab gets new functionality every month.