Good question, um, so-so question. Pleading guilty to owning a website that is not completely altruistic in content and management .. the answer would be, "To show up better in the SERPs." 10% of my pages are predominantly affiliate oriented and I am trying to get to #1 for my keywords without spending all my income on PPC (actually, I don't spend any at present).
That being said, what are "valuable links"? Links that bring visitors and no PR? Links that bring PR but no visitors? (sorry, "both" is not an option in this question)
How should I/we define what is a link worth having?
Is a PR2 that is from a "ho-hum" site "valuable"? What if that link costs a reciprocal link from a PR2 links page?