Domain redirecting is a classic SEO (be it black hat SEO) strategy that was used to rank websites. This was typically done by having microsites or buying expired domains and redirecting these domains that had strong SEO authority signals to a targeted area on a website.
Previously, this would generate an increase in the SEO performance of a website. And even now amongst SEOs, it is still a tactic that does work. However Google has cracked down on this method. There are 2 sides to the coin though. The first is that Google cannot really tell if you are redirecting a legitimate site in your website’s portfolio. And you will still receive the SEO benefit of the site. The other side of the coin is that you will get caught and your website will be punished with a ranking or Penguin penalty.
It is not a tactic that you should consider at all if you are seeking to engage strictly in white hat SEO activity. Black hat SEOers would usually use this tactic on throw-away domains. Meaning that if something was to happen to the site, they would just throw it away and create a new one.
Google encourages website owners to improve their SEO development by creating good quality content that will gain organic links and then Google will rank your website naturally.