Most people use shared storage, like iSCSI, for their XenServer storage repositories to hold virtual machines. Virtual machines *MUST* be in a shared storage repository in order to enable XenMotion and other advanced features.
It is handy to have some local storage repositories available, mainly to hold backups of machines. For example, you could shutdown a running virtual machine, right click on it, select Copy, and select a local repository to hold the copy. Then if you lost your iSCSI server, you’d have a local copy of the virtual machine you could start until the the iSCSI server became available again.
The XenCenter management GUI doesn’t allow you to make local storage repositories. You have to use the command line. Luckily, it’s pretty easy. Here’s my updated XenServer wiki page on how to do this.