Traditionally, we would run Jenkins in its own server. Even if we'd choose to share server's resources with other applications, the Deployment would still be static. We'd run a Jenkins instance (with or without Docker) and hope that it never fails. The problem with this approach is in the fact that every application fails sooner or later. Either the process will stop, or the whole node will die. Either way, Jenkins, like any other application, will stop working at some moment.
The problem is that Jenkins has become a critical application in many organizations. If we move the execution or, to be more precise, triggering of all automation into Jenkins, we create a strong dependency. If Jenkins is not running, our code is not built, it is not tested, and it is not deployed. Sure, when it fails, you can bring it up again. If the server on which it is running stops working, you can deploy it somewhere else. The downtime, assuming it happens during working hours, will not be long...