Differences between dependencyManagement and dependencies in Maven

What is the difference between dependencyManagement and dependencies? I have seen the docs at Apache Maven web site. It seems that a dependency defined under the dependencyManagement can be used in its child modules without specifying the version.

For example:

A parent project (Pro-par) defines a dependency under the dependencyManagement:

<dependencyManagement>
  <dependencies>
    <dependency>
      <groupId>junit</groupId>
      <artifactId>junit</artifactId>
      <version>3.8</version>
    </dependency>
 </dependencies>
</dependencyManagement>

Then in the child of Pro-par, I can use the junit:

<dependencies>
  <dependency>
    <groupId>junit</groupId>
    <artifactId>junit</artifactId>
  </dependency>
</dependencies>

However, I wonder if it is necessary to define junit in the parent pom? Why not define it directly in the needed module?

Dependency Management allows to consolidate and centralize the management of dependency versions without adding dependencies which are inherited by all children. This is especially useful when you have a set of projects (i.e. more than one) that inherits a common parent.

Another extremely important use case of dependencyManagement is the control of versions of artifacts used in transitive dependencies. This is hard to explain without an example. Luckily, this is illustrated in the documentation.