This is because the tasks involved with deploying applications and system administration have some significant overlaps. Capistrano is, essentially, a tool layer atop Net::SSH that can execute actions on a server, based on definitions written inside the framework provided by Capistrano. Capistrano was originally written for application deployment, but it wouldn't be "wrong" to use it for system administration. Sprinkle (a software provisioning tool) [1] is built on top of Capistrano.
1 - https://github.com/crafterm/sprinkle