1 - get free content from the writers.
2 - get free marketing and traffic from the writers (who tell their friends and follower about their article they've just posted on the Hufpo)
Most of the articles you write are all hidden in some sub-pages that no one but your friends or google seo would ever find.
edit: formatting.
It's all about the traffic.
They pretty much do anyway. Anything saying "CNN has reported..." is basically just a rewrite of the existing CNN article, to just enough of an extent that they avoid copyright issues. Do slightly better SEO (easy because huffingtonpost.com is more popular than cnn.com) and you get all the clicks. The organisation that spent money on the original reporting gets nothing. Yay.
Rarely do the reblog sites add much other than op-ed commentary to original news item and it'd save me time when someone sends me a link to a news item to root out the source.