It’s basically generic authority / ranking power – often called Link Juice or Link Equity – and for the past decade it has been gradually declining in significance as Google’s bounce-rate analysis has been improving, but PageRank still has a significant impact on rankings today.
The value of links is very much in the pure, quantitative PageRank; but it is also in the transference of keyword relevance (which is weighted by PageRank).
Is PageRank sculpting ethical?
Something to note is people in the white-hat scene, especially big brands with a reputation to consider, often snarl at the idea of PageRank sculpting, because Google advises us to focus on users rather than robots, thus PageRank sculpting is kind of a grey-hat tactic, but Google says a lot of things to deter non white-hat SEO — this doesn’t mean only white-hat works though. Google even includes a PageRank sculpting feature within its heavily promoted XML Sitemaps technology.
The best approach
The idea of being user-focused vs robots-focused is a good one for white-hat sites that don’t demand an efficient, effective, well organised SEO strategy; but the ultimate approach where organic search performance is as priceless as human user satisfaction, is to consider users and robots as one, and account for crawler flow in the same way as we account for user flow around our site. If the content is important, both crawlers and humans should be exposed to it fast, by following as few a links as is logistically convenient within an efficiently designed site.