Why "aCase" rather than "Case" in modules.php?

Hello everyone,
    I am using SugarCRM 6.5.x CE version. I want to ask this question to everyone from official SugarCRM personals to everyone who uses SugarCRM in a daily life. In include/modules.php file why did they mentioned
$beanList['Cases'] = 'aCase';
rather than
$beanList['Cases'] = 'Case';
this, like any other modules mentioned in that file?