This might consist of anything from the date the package was installed, to a list of files it installed on your system. Being able to access this information in a variety of ways can be helpful, too. Instead of simply finding out what files a package installed, it might be handy to be able to name a particular file and find out which package installed it. If this amount of detail is possible, then it should be possible to see if the way a package is presently installed varies from the way it was originally installed. Being able to tell if one or more packages are missing files could greatly simplify the process of getting an ailing system back on its feet again.
Files containing configuration information can be a real headache.
If it were possible to pay extra attention to these files and make sure any changes made to them weren't lost, life would certainly be a lot easier.