Two example use cases:
* you have a list and want to assert that there is exactly one element in it, and at the same time get that element: take a look at
* you have e.g. a list of invoices and want to get those invoice's common invoice partner: take a look at
Note: when using CollectionUtils frequently, it might make sense to statically import the class, i.e.
import static de.metas.util.collections.CollectionUtils.*;