It's another way for you to increase your monthly earnings. Try to read this: https://www.google.com/adsense/support/bin/answer.py?hl=en&answer=20012
Its placing Adsense in your RSS Feeds. So that when people read the feed. There will be Ads. And if those Ads stream into where you choose Feeds to display it could mean a better chance of clicks from where I see it. Or subscribers could unsubscribe because they don't want ads. Thats the way I see it.