I don't understand the difference between linked questions and related ones on Stack Exchange. Are linked questions only put there for the sake of SEO? Does linking questions inside SE (or any other website) have an impact on SEO?
Internal links are always good because they allow page authority to spread around a website.
The more internal links a page has, the more likely it is to be seen by Google as interesting and worthy of ranking. It is the same logic that applies to followers on Twitter or Facebook; the more real ones, the more likely a page is of interest.
As per Matt Cutts, Google Spam Control Manager, describe in this video on the account of Google Webmasters:
Internal links are very good for all websites and everybody should have to do this.