Google strongly advises that all links to your site should be natural and not paid for if they are to have any weight in terms of the search engine optimisation of your website. Links can help towards this as they essentially count as a vote for your website and the search engines like to promote popular websites.
The truth of it is that many SEO companies will actually buy links in order to build this profile up. Years ago this was done by purchasing hundreds if not thousands of links regardless of where the websites were hosted and if they were relevant but recently marketing companies have realised they need to be a bit more choosey in where these links are placed in order to compete.
Directories were often used to gain links back to your website but these are often not as valuable now as most have a “no follow” policy that means the search engines will essentially disregard the link and also often they are not high quality or relevant.