1 – because they alert us to your website when its new, or to new content
2 – because they are a vote of confidence in your site - quality websites tend to link to other quality websites
3 – because those links can send you direct traffic
4 – because over time, they can help establish a footprint that points to your authority on a topic (think guest blogging)
The main point from the post is pretty much: links are not everything when it comes to ranking in search engines. Nothing new there. Still, it never hurts to listen to the policies as they're explained by the search engines themselves.
On how many links you need, Forrester says, "Not as many as you may think. Again, as with so many other areas of search optimization, there’s no exact number here. On popular phrases with lots of query volume, to rank well will require more links from trusted, quality websites to boost your rankings. Less popular phrases can often require many less links pointed at your site to see the same lift in rankings. This is where a targeted link building approach can pay off for you."
The take-aways of the post, Forrester says, are: don't buy links, great content builds links, prove to users you're a trusted authority (and links will follow), and social media can help grow links.