Integration with Atlassian stack, you can:
* view builds and releases that are related to specific issues
* run builds by releasing out of JIRA directly
* display build/deploy reports in JIRA and Confluence
* view code changes and reviews that are related to your builds
It also has excellent support for Maven out of box, making it very easy to:
* configure build plans for projects that use Maven
* release codes that uses the Maven release plugin
There are also 3rd party add-ons you can use (for on-prem) for things it does not support out of box, or if you want, create your own add-ons.
Outside of Maven, its support for other build tools is not as good. Especially when it comes to making releases. Because it uses the maven-release plugin, there are some "magic" it uses, which is not available in other tools, and you will end up writing command line scripts to work around it to have the same result.
End to end build automation and deploying to various environments with different configurations. Allows for a streamlined build, release, deploy cycle.