If you want to learn a new operating system like Ubuntu, Fedora, or any other Linux or you want to play with Windows files by doing some alterations then you should go for Virtual box. You can easily learn by installing different Linux OS's on Virtual box. You may feel an irritation when you install Ubuntu as second operating system on your hard drive. You can practice on Ubuntu using Virtual box because it provides same conditions as it were real.
In other Conditions if you play with windows files installed on your main hard drive then it may cause some time removal or alteration of windows file that may lead your system to unbootable condition.