Why is package-info.java useful?

It is used to generate javadocs for a package.

/**
* Domain classes used to produce .....
* <p>
* These classes contain the ......
* </p>
*
* @since 1.0
* @author somebody
* @version 1.0
*/
package com.domain;

Will generate package info for com.domain package:

Example result: https://docs.oracle.com/javase/7/docs/api/java/awt/package-summary.html

Not only some findbugs annotations, but a lot of java annotations in common libraries have the java.lang.annotation.ElementType.PACKAGE type as one of the possible values of their own java.lang.annotation.Target annotation, e.g.:

com.google.gwt.core.client.js.JsNamespace
com.querydsl.core.annotations.Config
com.sun.xml.bind.XmlAccessorFactory
groovy.transform.BaseScript
java.lang.Deprecated
javax.annotation.Generated
javax.xml.bind.annotation.XmlAccessorOrder
org.hibernate.annotations.TypeDef
net.sf.ehcache.pool.sizeof.annotations.IgnoreSizeOf
org.apache.hive.common.HiveVersionAnnotation
org.apache.wicket.authroles.authorization.strategies.role.annotations.AuthorizeAction
org.codehaus.commons.nullanalysis.NotNullByDefault
org.eclipse.persistence.oxm.annotations.XmlNameTransformer
org.glassfish.jersey.Beta
org.jgroups.annotations.Experimental

and much more.

This package-info.java file would be the file, where you can place such annotations (along with the javadoc).

Annotations

Another good reason to use package-info.java is to add default annotations for use by FindBugs. For instance, if you put this in your package-info file:

@DefaultAnnotation(NonNull.class)
package com.my.package;

then when findbugs runs on the code in that package, all methods and fields are assumed to be non-null unless you annotate them with @CheckForNull. This is much nicer and more foolproof than requiring developers to add @NonNull annotations to each method and field.

A package-info.java file allows adding javadoc to document a whole package. See http://docs.oracle.com/javase/7/docs/api/java/applet/package-summary.html for example.

If you don't care about missing package documentation, then ignore the warning or disable the JavadocPackage check.