What's the importance of restoring SQL Server system databases (model, master, etc.)?

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http://serverfault.com – I had to restore some production databases to a different drive on the same Microsoft SQL Server 2005 machine. That worked fine and the application using the databases is back online. However, i have not restored the system (or default?) databases that SQL Server creates on its own (model, master, etc.). My question is, what is the role of these databases? and how important it is that i restore them? (HowTos)