The page mentions cryptically "They could almost be thought of as a self-executing container without the virtualization". The "self-executing" bit makes sense - you don't have to remember to type "docker". "without the virtualization" doesn't make sense unless they mean without cgroups or are talking about Kata Containers.
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.
[ed: I now saw this question and answer:
https://news.ycombinator.com/item?id=17526178
Frankly using system/external python (or other VM) seems a bit risky... But whatever works, I guess..]
One big benefit is that the filesystem only decompresses the pages as needed, which greatly improves the start up time over existing solutions.
Containers have virtual file systems they access instead of the host one. XARs don't.