Maven incremental build plugin enables maven2 to realize a pseudo incremental builds on multi-modules projects. With this plugin declared in the parent pom, the modules with modification and all their dependendent modules will be clean before any action on it like mvn compile or mvn package.
No needs to call mvn clean compile on parent pom to ensure that the entire project compile.

You can see here a good french article on maven-incremental-build advantages.

See the maven generated site for stable version for more details.