In my speaking I talk of the two extremes of blogging. On one end of the spectrum, you have folks like me who share what they read, adding a comment or two to a block quote from another post. We're intelligence agents on a niche topic.
180 degrees on the other side of the spectrum, there are those who post original content without referencing other blog posts as sources. You'll find most large law firm blogs in this group.
Greg Knauss on Kottke.org defines the two groups as referential and experiential bloggers.
The referential blogger uses the link as his fundamental unit of currency, building posts around ideas and experiences spawned elsewhere: Look at this. Referential bloggers are reporters, delivering pointers to and snippets of information, insight or entertainment happening out there, on the Intraweb. They can, and do, add their own information, insight and entertainment to the links they unearth — extrapolations, juxtapositions, even lengthy and personal anecdotes — but the outward direction of their focus remains their distinguishing feature.
The experiential blogger is inwardly directed, drawing entries from personal experience and opinion: How about this. They are storytellers (and/or bores), drawing whatever they have to offer from their own perspective. They can, and do, add links to supporting or explanatory information, even unique and undercited external sources. But their motivation, their impetus, comes from a desire to supply narrative, not reference it.
Most bloggers are going to be a blend of both. They'll be citing other bloggers so as to network via the blogosphere while posting solely original content when appropriate.
From a marketing perspective, the referential style has some benefits. It gets you into the Internet discussion. By blogging about others' content, your content will get cited by them. That increases your exposure to your target audience.