What Impact Do Article Categories Have on Search Engines?

I've just discovered Google Reader and I love it and I'm in the process of moving my feeds over from Rojo.  However I've just spotted something that I didn't realise before.  Google Reader is picking up the categories that I am assigning to each article.

Previously I thought these were just for use within my blog.  Have I been missing a trick here and should I be optimising these as well?  Judging from the categories that the majority of the feeds that I read have been using, it looks like many people have been making the same mistake.

Can anybody clear this up?

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