Standard SEO which should be done by all. You'll notice that the links go b->A c->A d->A, but site A does not link back. Linking back raises red flags on SEO. Also, be careful creating a bunch of new sites and linking to a site, or getting many new links overnight.
A NEW site linking to a site will not help with PageRank, but does help toward a little bit of exposure. Google won't put much weight on it until the domain/site is established and has clout or rank to share with others. It's an UNTRUSED source until that point.