Stephen Hackett:

In short, the via link is a simple way to show your readers how you found the content you are sharing with them. The idea is to thank the resource you used to find the content you are sharing. As a bonus, it’s a nice way to toss some traffic at other people’s sites.

My criteria for via links are simple: If someone points me to an article I did not find on my own (either through my RSS reader or at its original source), I’ll include a via. Likewise, if an article I found on my own links to a second article, which I then post to my weblog, I’ll link to the original source.

It’s pretty straightforward. You know when someone helped you find something. And more often than not, they deserve a little bit of credit.