It's intended as a replacement for _scala_ builds. Having a build definition in the native language that doesn't require a different syntax (like a declarative syntax such as maven xml or toml) makes task customization easier for the maintainer of a given project. Unfortunately, it also means that you have to know the language and read the documentation for the build system.
If you want something declarative, there's also bleep[1] in the scala ecosystem. And for single module builds there's scala-cli[2]. It's also possible to use gradle and maven for scala projects, but for an java-only shop I wouldn't be using mill or bleep because there's no need to introduce a new language just to manage the build. For scala/java/kotlin hybrid projects though, gradle or mill or sbt would be my recommended tool because of how tightly they are coupled with the cross-platform build matrix nature of scala library and build system plugin ecosystems. For larger builds, it's mill or bazel because there s a performance cliff in sbt and gradle, and bleep is too new to have all the standard plugins ported. We use mill at writer.