General
A way of both organizing and configuring your project. It's essence it that a basic environment (only Java, maybe a build tool, and your IDE of choice) plus the project source code is everything you need. To read more, there is a page dedicated to it.
Because it allows new developers to start coding immediately, without needing to bother you to get their environment working. To read more, there is a page dedicated to it.
Not yet. In the world of Java, it's increasingly common to have an Instant Developer Experience, but many people are not quite there yet. Some require, for example, require more than just the language and a buiild tool and insist on additional tools to be installed (such as docker, docker-compose, helm, etc. These extra installs are cumbersome when you just want to get started.