there are indeed some embedded databases which work directly on files or something like this and don't use a server. But usually there IS some kind of server and that's true for Oracle and MySQL, too. Maybe you just didn't notice that because you used an installation utility which installed the database server transparently for you on your local machine as a Windows service or something like this.
With MongoDB there is no difference at all and of course it's fine to have a MongoDB server running on your local machine for development or testing purposes. Just because there is a "server" doesn't mean this server has to run on a different machine!
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Building a Better World in your Backyard by Paul Wheaton and Shawn Klassen-Koop