If you have larger projects it happens that you have test classes or helper classes for unit tests which you would like to reuse in different projects of your own.
Let us take a look at the following projects m1, m2 and test. The projects m1 and m2 are created within your company but otherwise they are unrelated. So there some basic setup's for you projects which are always the same. So you decided to put them into a separated module test, but now the problem occurs that you like to access those classes from the other modules.
m1 m2 test (common unit test helper)
In a multi-module build you can reuse unit test class in the same way.
pom.xml m1 m2 test
<project ...> [...] </project>