The content property goes hand in hand with the :before and :after pseudo elements and allows for content to be added to a document through css. Quote marks for blockquotes and the q element are typically generated by the browsers and added via the content property.
IE6 and 7 however do not understand this and in order to maintain a level playing field the reset removes the quote marks from all browsers which then allows you to add them direct to the html.
Eric makes the point that generated content should not be used for important content and if a quote is important to the context then you should apply it in the html and not in the css.
I’m very leery of reliance on generated content. If you rely on generated quotes as recommended by the W3C, then in any non-CSS user agent, there will be no quotation marks. I would rather suppress generation of quotation marks and insert my own, the same way I would avoid or suppress automatic generation of list counters and have them in the content if they were crucial to the document’s integrity. (For example, legal codes.)