Generally XARs are lighter weight than a container. While you can (and sometimes we do) use XARs to deploy, say, a self-contained service like a website, often they are used to replace command line tooling.

Container isolation (cgroups, namespaces, etc) would make it difficult to do some of the system level tasks we use such tools for, such as configuration changes or monitoring.

Likewise, we often are replacing a PAR or C++ tool with a new XAR version, and it is nice to simply replace the executable and not have to change how it is invoked. In this regard, invoking a XAR is identical to running any normal executable or shell script.