The place where I typically used DefaultValue is for classes which are serialized/deserialized to XML. It does not set the default value during instantiation and doesn’t impact autoproperties.
From MSDN:
A DefaultValueAttribute will not cause a member to be automatically initialized with the attribute’s value. You must set the initial value in your code.
MSDN – DefaultValueAttribute Class
Edit: As Roland points out, and as others mentioned in their answers the attribute is also used by the Form Designer