Maven's '-r' command line switch builds an implicit reactor by recursing through subdirectories of the current working directory.
On the face of it, this is a useful facility, especially valuable when bootstrapping a new project. You do not need to maintain a 'root' pom.xml for a project with explicit modules directives. Instead, you simply use -r and Maven can order and invoke the build for each module within the project.
However, I now avoid over-reliance on the implicit reactor. My reasoning for this stems from the release plugins inability to release atomically across a set of modules using the implicit reactor. Instead each module must be individually released. In large projects this is simply impractical. At least one large shared-source project known to me switched its build from the implicit to an explicit reactor - to facilitate the release process.